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Thread: Compression confusion

  1. #1
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    Compression confusion

    Hi All; New member, first posting. I'm looking forward to participating with others. I have a 1973 R75/5 (actually 1972 1/2, with longer swingarm). I bought new in Germany (domestic model, not export), currently has approximately 95,000 miles, all put on by me. I did through prepping in 1985 (change all fluids, oil in cylinders, empty gas tank and carbs, removed battery). Has always been garaged and kept under cover since. No rust, all original parts (except for SS mufflers which match originals design). I looked into cylinders with scope and no rust visible, I have very slowly hand turned it over and there are no hangups. I am planning to have all items refurbished, i.e. all engine seals, forks, transmission, electrical, etc. Keep the original points/condenser ignition configuration. Since I don't have the proper tools (or expertise), I will have a local shop do the work and register it as an antique here in Texas, no major, long trips. My question and looking for some experienced opinion is; since it should have premium leaded fuel (min. 99 ROZ) which is no longer readily available, which is the better option;
    - when the engine is being rebuilt, have a thicker cylinder base gasket installed, thereby reducing the existing 9.0:1 compression (and dealing with the associated valve tappet adjustments and timing issues) OR
    - use an octane booster/lead additive ( I have read the carburetor floats can deteriorate due to some additives ) OR
    - any other suggestions/recommendations...
    Thank you all in advance.
    John
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  2. #2
    Liaison 20774's Avatar
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    John -

    Welcome to the forum! Nice that you are the one-owner for the bike...kinda rare these days. Not sure how you have it titled, but the longer swing arm for the /5s came out in the latter half of the 1973 model run, so I think it's a 1973. But that's just me.

    I'm not sure about the ROZ designation...maybe that's a European thing? I'm aware of the RON which is research octane number and MON which is motor octane number. Here in the US, the pump numbers tend to be (RON + MON)/2.

    As for your bike, the specs indicate it was 9.0:1...my R100/7 was 9.1:1. I've always run the most premium gas I can get, here in Texas that's 93...bike has always run fine. I recall once on a trip to Death Valley, the only gas I could get was 87 and the bike did not like that at all! I have since done a top end and put a Siebenrock kit on it which raised the compression ratio to 9.5:1. I didn't want to consider dual plugging, etc., so I installed the base shim used for the R60 series which effectively dropped it to a little below 9.0:1. I still have decided to run premium in the bike.

    I would forge ahead with your plans without a base gasket and run Texas premium in the bike. You could try dropping back to the mid grade and listen to the engine to see if it talks back after a couple of tanks. I'd stay away from additives...probably just a drain on your wallet.

    I presume the rebuild will include a new top end...BMW parts for the '85-on Airhead models have the proper valve seat and valve material to handle unleaded gas.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User lvermiere's Avatar
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    Quote Originally Posted by 73bemr View Post
    Hi All; New member, first posting. I'm looking forward to participating with others. I have a 1973 R75/5 (actually 1972 1/2, with longer swingarm). I bought new in Germany (domestic model, not export), currently has approximately 95,000 miles, all put on by me. I did through prepping in 1985 (change all fluids, oil in cylinders, empty gas tank and carbs, removed battery). Has always been garaged and kept under cover since. No rust, all original parts (except for SS mufflers which match originals design). I looked into cylinders with scope and no rust visible, I have very slowly hand turned it over and there are no hangups. I am planning to have all items refurbished, i.e. all engine seals, forks, transmission, electrical, etc. Keep the original points/condenser ignition configuration. Since I don't have the proper tools (or expertise), I will have a local shop do the work and register it as an antique here in Texas, no major, long trips. My question and looking for some experienced opinion is; since it should have premium leaded fuel (min. 99 ROZ) which is no longer readily available, which is the better option;
    - when the engine is being rebuilt, have a thicker cylinder base gasket installed, thereby reducing the existing 9.0:1 compression (and dealing with the associated valve tappet adjustments and timing issues) OR
    - use an octane booster/lead additive ( I have read the carburetor floats can deteriorate due to some additives ) OR
    - any other suggestions/recommendations...
    Thank you all in advance.
    John
    On my new 75/5 that I bought in June 73, that I still have, all I did was have the no lead valves installed in the heads and put it back together with new rubber lower oil seals.

    Still starts and runs as new with 130,000 on it. I a 1200RT now but the /5 I fire up every few years.

  4. #4
    Registered User STEVENRANKIN's Avatar
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    My experience

    I have two airheads, a 78 European R100RS with the 9.5 to 1 compression and a 84 R80RT with an engine rebuilt to European specifications of compression 9.5 to 1.

    Both bikes run very well on 93 octane 10% ethanol fuel. I DO however try to run non ethanol fuel which in my area is 90 octane. Both bikes run very well on this.

    I have been stuck more than a couple of times with the RT been at stations that don't offer 93 10% or 90 non. As such, I used the 89 10% and never have had a problem with pinging or running.

    The only time I have ever had problems with an airhead pinging was with my first bike a 77 R75/7. It would run beautifully for quite a few miles then start pinging. That was its way of telling me to get my self under the front cover and adjust the points or replace them. The points wearing changes the engine timing. In MY case the change was an advance in timing which led to the pinging. With new/or properly adjusted points and timing, I never had a problem with pinging.

    On my 78 RS I replaced the points with what was available at the time, a Boyer system. Now a day, there is a system that takes the points and turns them into a switch only, rather than passing a high current though them. The load is taken off the points and the last very much longer than when in stock mode. I can't think of the name of this system. Other airhead people know what it is and I am sure if you look hard enough, you will find out more about it. Also, I believe Motorrad-electrick, Rick Jones, offers a complete system to replace points with a Hall effect system similar to the BMW system introduced in the newer bikes.

    If you go to the current thread Brook Reams has regarding his rebuilding a 83 RS, he installed a completely unknown to me system for ignition.

    Alright, I have bored you with ignition things the you asked for compression information. I have friends with R90S bikes and the 10 to 1 compression who have no problems with pinging on the highest octane gas they can get. I believe you will be able to rebuild without lowering the compression and will (unless they mess and change the available gas down the road) be able to ride with out a problem. Don't give up the couple hp and torque, at 45 to 55 hp, you will wan them. Cheers, St.

  5. #5
    Airmarshal-IL James.A's Avatar
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    I also own a Long Wheel Base R75/5. It's titled as a 1973 and the build date on the VIN plate indicates 12/72. I have owned it since 1989. Before having the valves and seats converted for un-leaded fuel, I always used the highest octane I could find. Performance would suffer when compelled to use low grade gasoline (87). Now that the heads are upgraded, I do not hunt for ethanol free fuel, but still run the highest octane I can find. Now, whenever I must use low grade fuel, it doesn't seem to matter much.

    The floats will deteriorate over time. I judge float viability by color. Yes, I have seen floats turned black in at least 1 Airhead. I like to replace them in my bike when they get to medium amber.

    I am not a fan of thickened base gasket shims to reduce compression.

    As you surely know, an R75/5 is a fine machine. If you choose to upgrade the valves and seats, I recommend using OEM parts. I had a bad experience with non- OEM valves
    1973 R75/5

  6. #6
    Liaison 20774's Avatar
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    I think what Steven is referring to is a points booster. I don't see anything on Motorrad Elektrik but Euromotorsport has one by Dyna. Just google points booster BMW and you'll see some discussion.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #7
    Registered User ebeeby's Avatar
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    The 73 R75/5 here had terrible valve recession problem at 38,000 miles when I bought it in 2005. I had Ted Porter's shop re-work the heads with new seats, valves, guides and springs. From my experience, some folks experience it and some don't. The greatest symptom is the valve lash closes up to a point that compression suffers and hard starting ensues. Adjusting valve lash then gets to be a regular exercise. At this point, the only solution is new lead-free gas capable valves and seats.
    Where in Texas are you?
    1973 R75/5

  8. #8
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    Compression Confusion

    thanks to all for responses, it's GREAT to have such a sharing community! The only engine work that has been done is new valve guides at approx. 75,000 mi. (mid 1970's) from Perry Bouschon/Fr. Worth. It has NO leaks but the local N. Dallas dealer recommended I do a total engine refurb since all seals have probably dried out and would eventually leak (and onto clutch) thereby requiring new clutch, etc. After going thru dealer reviews (granted, all reviews are taken with a healthy dose of salt), may go with the Ft. Worth dealer. Anyone with experience there or other recommendations? Since it will be registered as an antique and therefore can (legally) only be ridden in parades/ceremonies/shows/etc. therefore anticipating putting on only low future miles, I feel I should go with upgraded valves/seats and use highest octane fuel available. PS. I'm in the Mansfield, TX area.

  9. #9
    Liaison 20774's Avatar
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    I have a couple of bikes registered in Texas as antiques...even have year-of-manufacture plates on them. Don't get hung up on the show/parade/etc restriction. I can't imagine an LEO is going to find the time to pull you over and confirm. I was pulled over for crossing a double white line in a small town...jerk in front was doing goofy things...I wanted out of there. Cop gave me a warning...didn't once question my antique status. I think YOM plates don't come across as obvious antique whereas the Texas-issued antique plates spell it out directly on the plate.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  10. #10
    Registered User STEVENRANKIN's Avatar
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    Follow up

    I think what Steven is referring to is a points booster. I don't see anything on Motorrad Elektrik but Euromotorsport has one by Dyna. Just google points booster BMW and you'll see some discussion.

    Yes, right on the head a points booster. Lots of guys swear by them.

    As for Motoradd-Eelectrik, they make a complete system for the early bikes to replace points and advance system with a Hall effect sensor and black ignition module found on the newer airhead bikes. I believe it is the Alpha system. For the guys with points in a can, the 79-80 bikes, they make a system to replace that with a system very much like the point less system started in 81.


    If you are going to only ride a few miles a year, the stock points will be fine as well as new stock valves. The valves available now are treated differently to make the fast recession a thing of the past.

    I believe replacement carburetor floats now are alcohol proof as well.

    Your bike will run fine, the R75 is one of the best engines BMW made. Just don't be cheap and buy cheap low octane gas. Enjoy the riding. St.

  11. #11
    Registered User ebeeby's Avatar
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    I have had Texas antique plates on several vehicles for nearly 20 years. I drive them whenever and wherever I want. I have never been stopped, questioned or in any way challenged about them. In the antique plate description (when I first signed up in Texas) was a phrase which I remember as "....and other functions of public interest." and " ....will not be used for daily transportation." Well, I meet both of those! It has always been on the public's interest to see antique vehicles on the road!

    Perry had an old airhead guy at his shop but he died I think 15 years ago. I don't know how they ever replaced him.

    And seals don't "dry up". they are stored on shelves, often for years, in the open air and don't dry up. They do of course deteriorate with constant heat cycling and with exposure to ozone etc. But I see no point in replacing seals that are not leaking. There are exceptions - for example the oil pump seal is a candidate if you remove the clutch. That's one of those "while you're in there" deals. And when was the clutch last replaced (or measured, if ever) on this 95,000 mile bike?

    There are shops known to do quality and correct head work, and there are shops that claim to do head work. I would send the heads to one of those known shops such as Ted Porter's.

    If the motor turns over by hand, I'd replace the crankcase oil and filter, fuel it up, put in a battery and see if it fires! (given that you prepped it all those years ago). If it runs and will take a carb balance, I'd replace the tires and ride it to see what you have. See what leaks. See if the valve lash closes up and the heads need re-work.
    1973 R75/5

  12. #12
    Registered User STEVENRANKIN's Avatar
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    Reading problem

    I have to admit, I am have a problem occasionally when reading forum questions not to read them properly.

    So, now I will say; the question posed to open the thread was regarding a bike with 95000 mile on it, It had been placed into long term storage and this was done properly.

    Cutting to the chase, put a battery in it, start it up, and ride it! IF seals have gone, worry about them next winter unless they are gushing oil or putting oil on the tire or clutch.

    If you have good compression, then don't bother at all tearing the engine apart to rebuild. You said you looked into the cylinders and did not see rust, same again, don't bother a tear down unless you have low compression. Perhaps check the valve gap now and keep an eye on it as you ride. Valve recession will show in the exhaust valves tightening up quickly or faster than the normal 15k interval of adjustment. If the valves stay in adjustment, don't bother a rebuild unless you have low compression.

    If it were my bike, I would do something about the points system to improve or replace them. They were the only thorn in my side when I had my R75. You can go simple with a point booster or full electronic. Of course, if the bike is only going to be ridden a few miles a month, stock points will do fine as long as they are adjusted.

    BMW airheads are pretty darn bullet proof and will run fine under some of the most horrible conditions. Believe me, I have seen this at my friend's shop. As for me, I managed to get over 100K on my R80RT before I had to have the engine rebuilt, and I was riding the hell out of the bike at the time. Since I have slowed down with age, the second rebuild will most likely be done by the next owner. I am at 207K and counting.

    Don't spend money unless you have to. Don't be cheap if you do.

    As for the fellow who wrote about the oil pump seal, I worked on a bike where the owner replaced the oil pump seal he thought was leaking, put the flywheel on in the wrong position and the next owner my friend and I ended up replacing the leaking rear main seal. Of course, we put the flywheel back into proper place. So yes, when changing the two seals in the back, do both at the time.

    Enjoy riding the bike regardless of restrictions on registration. The 750 engine was one of BMW's best. Cheers, St.

  13. #13
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    Many thanks to all ! As suggested, I've decided to not do the engine rebuild; just change oils, repack bearings, new tires/tubes, battery, crank her up, see what happens and enjoy a ride! God bless all!!

  14. #14
    Registered User STEVENRANKIN's Avatar
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    By all means if you can

    Just for the sake of things by all means If you have a good compression tester, go ahead and see what you get for readings.

    Of course, I have seen and had a couple of bikes with shot rings and blasted pistons and jugs that still ran fairly well all considered. It never ceases to amaze me how much wear an airhead will take before they protest the treatment.

    In my case, I started burning oil on one bike supposed low milage bike that in fact had worn rings. I don't know the exact mileage on the bike but I assume the odometer has been round the horn once at least

    The other bike started clattering and it was because the rings were shot and the piston bases were slapping the wall of the cylinders. It had 120K miles of hard use from me, so it was no surprise.

    Oh well, enjoy riding, Cheery bye, St.

  15. #15
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    Compression Confusion-Update 6-29-20

    Since everyone was so forthcoming with the good advice. Thought I'd give a short progress report. So far;
    - I've removed the 2-headlight café-racer style fairing (very stylish in the '70's) and did the necessary re-wiring
    - Re-installed original horn and blinkers
    - Removed the front and rear wheels; removed the glazing on shoes and drums, repacked bearings, installed new rim strips, tubes and tires (36+ year old tires are NOT very flexible!! After struggling an hour, had to cut the cord on the rear tire to get it off the rim).
    - Changed all fluids (forks, engine, trans, driveshaft, rear drive), oil filter. Old oil was black... to be expected. Air filter like new.
    - New spark plugs (have new points and spark plug wires, not yet installed).
    - Oiled all control cables
    - Greased all nipples
    - Installed new battery. Instrument lights come on . Engine turns over with starter (just enough to see if it will work)
    - Put enough gas in the tank (added 'lead substitute' as a precaution)
    Now comes the 'fun' part...
    - Reinstalled original petcocks, one leaks (ordered 2 replacements)
    - Right carb leaks
    - Removed both carbs and tested (the right carb Float Needle Seat isn't sealing). OK, since both carbs are off, might as well go completely thru and do a thorough cleaning. Many thanks for recommended web-sites, EXTREMELY helpful. Ordered re-build kits with new Float Needle Seats. Disassembled carbs and currently cleaning. Good thing, they both were clogged in various places. Currently the only hick-ups are;
    Screws holding Throttle Slide Cover top on won't budge on either carb (if I blow into the intake, the Throttle Slide raises and lowers on both carbs, so I'm 'assuming' the diaphrams are functioning correctly). The left carb internals disassembled and are cleaning easy enough. The right carb Idle Mix Tube is a BEAR!! Soaked everything in Berrymans B-12 and WD-40 Rust Release Penetrant (made sure NOT to get the diaphrams wet) but 1/2 of the Idle Mix Tube broke, uggg!! Still soaking, afraid if I turn too hard the other half will break off, then what??? . Obviously will have to order a new tube. Anybody have any suggestions? Thanks ahead of time.

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