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1972 R75/5 Running Poor Acceleration

jcurt325

New member
1972 R75/5 Running Poor Acceleration - SOLVED

Greetings all, new member. Just did my newbie post in the new members section. Have a question about my '72 R75 and an issue with sluggish acceleration. I'm new to BMW (coming from Honda SOHC world) and i'm just getting the hang of it. Have gone through the bike overall to check things out, replacing missing or incorrect items here and there (clamps, plugs, etc.). Bike appears fairly unmolested but not much known about its history so i've done the maintenance items changed oil, air filter, trans fluid, drive shaft and final drive fluids. New petcocks and fuel lines along with tires.

Bike has been cranking and idling well (I believe having NO idea how well one of these idles); however, its sluggish on acceleration especially at cruising speeds (around 60). I understand these bikes are not quickest or fastest around and i'm not looking for that. After warm up of ten minutes or so, the bike hesitates and shudders with any significant twist of the throttle. However, while twisting the throttle and reach down and put the choke at about halfway the bike immediately smooths out, accelerates very strongly and takes off like a champ. It will cruise like this perfectly until you of course come to a stop and then its idling in the 3k range. Pull off the choke, bike drops down to a few low and steady idle, but if you go to accelerate again there is sluggish response.

I'm coming from the world of Honda SOHC's and at present in the dark about the Bings. While I get up to speed I took the bike to a local shop that I have relationship with, and coincidentally the owner rides a beautiful R90S. He gave the bike a once over for me and set the timing, rocker clearance and endplay and checked out the carbs. He said the diaphragms were good and passages all looked clean. He synced and dialed it and it is cranking and idling beautifully. He told me to try it out and see how it was running and bring it back if any issues and he'd sort it out without charging me again. He did not do a full clean of the carbs or a rebuild, as he was wanting to try the other things first just to make sure and save me some $$ on rebuild parts if not necessary.

So, I'll be taking it back to him with the findings of the ride this past weekend, but I wanted to come to the well here to see if anyone had thoughts. I'm assuming it's probably needing a carb rebuild at this point? It just seems odd to me that its running generally so well otherwise.

An additional odd thing regarding the throttle and i'll do my best to not butcher my description of this. There is a point towards the end of the rotation of the throttle that I continue to rotate the throttle the engine actually decreases in rpms or bogs down further. It's hard to tell if its just slowing back down or the mixture is going exponentially fubar that it bogs further. So, put another way, as I'm twisting the throttle the engine revs increase in step with the twist/opening of the throttle to a point near the end of travel for the throttle. But then as the throttle is opened beyond that point the revs fall off for those last maybe 10 degrees of rotation. I understand that's a horrible description. I'm not sure if this is problem with the cables or throttle assembly or if related to the overall poor running but i throw that out there too.

Thank you for indulging the rookie questions.
 

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First thing I would do is purchase the BING book (from Max's BMW) - I think it goes for around $12. It had detailed explanations of how the bings work, and it has several pages of Bing applications on BMW's that give you standard stock settings.

Now, having said that, it sounds as if the carbs' mixtures are great for low end performance, but somewhere along the line, they sound like they are going lean (since choking seems to help). Personally, I would take them apart one at a time, and be sure not to mix parts between them. I would then think about replacing the Jet Needle, and the Needle jet. Also, simply make sure that the floats are floating and properly adjusted. Make sure they are actually shutting off the fuel flow as they are supposed to do.

I also am wondering about the spark adjustment and then the spark advancing as it is supposed to. If it doesn't, it will be sluggish at higher speeds.

I am not an expert, but this may help. There are certainly others who know more than I do.

Other things I would check on the bike since it is new to you. BE SURE to remove and re-pack the wheel bearings. I was told my purchased bike was pretty well maintained. I found LOTS of things that weren't. But the most expensive and most dangerous were the front wheel bearings. DRY as a bone! I was lucky I checked them!

Check and clean all battery connections. Also, if removing the front engine cover, be sure to disconnect battery first so you don't accidentally fry the diode board up there. Check and clean any electrical connections on the ignition and charging systems.
 
R75/5 Running

My suggestion if you are new to Airheads, would be go to Snowbum's site and do some reading.

It is an excellent source of information.

http://bmwmotorcycletech.info/technical-articles-list.htm

The suggestion of getting the Bing book should be taken. It is also available if you call Bing.

Another suggestion is to join airheads group.

http://www.airheads.org

There are many of us around the country/world who have rebuilt these wonderful machines.

Your profile doesn't say where you are??? There are no doubt some local Airheads who would be glad to lend a hand to get your machine sorted out.
 
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First off, congrats on the airhead and welcome to the MOA jcurt325?

I am relatively new myself but have learned the very last thing to adjust or mess with is the carbs. Re check valve lash and timing. Check to be sure the air intake is not plugged up. When you replaced the air filter did you check to see if there were any obstructions (say mouse nets) under the starter cover and in the air suction hood? Check spark plugs to be sure they're still clean and check that the throttle cables have proper free play too.

Good luck and keep us posted.
 
When I heard the bogging down at 60 or so, my first thought was diaphragms. But if they've been checked and inserted into their proper detents in the top of the carb body, then maybe they're OK.

I like the idea of the floats and the levels. For these 32mm carbs, IIRC the height of the fuel when measured in the center of the bowl is around 24mm. This is easily checked by letting the bowls fill up, turning off the petcocks, and carefully removing the bale wire holding the bowl on. Then measure the fuel. If running lean (less than 24mm of fuel), you could be bogging down at speed, especially when rotating to wide open throttle...lots of air, no gas.

You could also have a misadjusted throttle cam. When the top of the throttle housing is carefully removed, you will see a ring of teeth on the grip side and a horizontal ring of teeth connected to a small chain. There are marks on both sets of teeth that must be aligned at zero rotation of the throttle. It could be that you have a misadjustment there...worth checking. Also worth putting a small amount of silicone lubricant on the meshing teeth.
 
Diaphragms are fairly cheap and I really go along with the idea that that is the problem........If it was me, instead of just checking them I would get a new set along with the rubber "O" rings for the jets and install them. There can be rips or tears in those thin rubber diaphragms that one doesnt see........ Won't take but a minute and what you desribe about falling off when going 60 or so and then catching up.....pretty much gutless with roll on......that is classic diaphragms.

I agree with Jimmy about checking the advance mechanism. Take it off and clean it and then grease it with the PROPER grease. Bosch Ft 1 v 4 Grease......A tube will last for your life..........

HUGE NOTE..............When removing that front cover to get into the points.....ALWAYS......take off the ground cable back at the transmission before removing that cover...........ALWAYS........If you don't we will soon be getting a thread about burning up the diode board and how to fix it............God bless......Dennis
 
, IIRC the height of the fuel when measured in the center of the bowl ...

I have been racking my brain, but what does "IIRC" mean? I understood your sentence and overall meaning, which was great, but can't figure out what IIRC means.
 
I have been racking my brain, but what does "IIRC" mean? I understood your sentence and overall meaning, which was great, but can't figure out what IIRC means.

Several meanings generally speaking; however, I would venture to guess in this instance it is used as a acronym for IF I RECALL CORRECTLY......
Hope that helps.
 
Thanks for all the pointers! I've heard the point about the battery and the front cover from a coupe of others who passed along their "unfortunate" wisdom. Bing book ordered Jimmylee. Will let everyone know how it progresses. Thanks so much.

Curt
 
Also, Radar41 - I updated my profile to add location. I'm in Savannah, GA. Thanks for reminding me, still getting settled in. Thanks all.
 
Airhead Help

Also, Radar41 - I updated my profile to add location. I'm in Savannah, GA. Thanks for reminding me, still getting settled in. Thanks all.

thanks,

Nathan Mende is the Georgia Airmarshal and can put you in touch with some serious Airhead help :).

Sent a PM with his contact info.

radar
 
And its ANSWER time!

Going through all the great suggestions....check the carbs, look for obstructions in air path. Air filter clear - check. Under the starter cover clear -check. Air intake scoop clear....wait, so clear in fact its missing and there's a giant hole in the top of the cover where it should be. Never got to the point of removing the tank to visualize it before. Previous owner didn't mention it being lost. So based on everyone's suggestions of a lean condition at speed and the fact that the enricher balances things back out, needed to reduce the hole and the air coming in and see what changes. Not having a replacement scoop on hand, couple strips of duct tape over the cover to reduce the opening for a test. And the thing runs like a scalded dog! Flies through gears. Needs no choke. Smooth and steady. Quick run over to Ebay and picked up a replacement intake scoop (and starter cover with all the clips for the scoop) for $25! I would think even in this test setup its still probably getting a little more air forced in than usual since the scoop is rearward facing and at present its getting pushed under the tank at speed. We shall see how it does with the scoop back in but i'm thinking we can call this one resolved!

Also, the issue of the throttle falling off at full open seems resolved as well by this, though full throttle is now not needed.

Thanks again everyone!
 
The intake scoop (and screen if one existed) shouldn't really restrict the air coming into the filter. It's pointed backwards just that it doesn't actually scoop hard particles directly into the top of the engine...theoretically, a hard particle will have too much momentum going backwards to make the 180 degree-ish turn back into the scoop.

I think you're experience has just demonstrated there's something wrong with the air-fuel mixture settings in the carb. But physically restricting the amount of air that could get to the carbs, you returned the balance to a reasonable number.

But certainly, go ahead and put the scoop on and see what happens...report back on that and let's see if that's the answer or not!
 
And its ANSWER time! Not having a replacement scoop on hand, couple strips of duct tape over the cover to reduce the opening for a test. And the thing runs like a scalded dog! Flies through gears. Needs no choke. Smooth and steady. Quick run over to Ebay and picked up a replacement intake scoop (and starter cover with all the clips for the scoop) for $25! I would think even in this test setup its still probably getting a little more air forced in than usual since the scoop is rearward facing and at present its getting pushed under the tank at speed. We shall see how it does with the scoop back in but i'm thinking we can call this one resolved!

Also, the issue of the throttle falling off at full open seems resolved as well by this, though full throttle is now not needed.

Thanks again everyone!

Not to be a naysayer, but I have a hard time visualizing how that scoop addition would restrict that much air! It is at the higher RPM's and that was where you were having the apparent lean-ness. That is where it will be sucking and needing the most air. Maybe the scoop will actually restrict at that higher use?

However, stranger things have happened.

I am anxious to see how this will work out.
 
Okay...weather finally got nice so could get back on things. Put the full replacement scoop in. Bike continued to run well, much improved similar to the high-rent tape repair. Was preparing for a long trip on the bike and doing a number of service items to get it ready for a long run (change all fluids, wheel bearings, etc) and so decided to go ahead and put in rebuild kits on the carbs to eliminate any questions. Didn't want to leave anything to chance to go wrong and be stuck in the middle of no where.

With rebuilds in, the bike run even better. Same characteristics as before just seems to be even better at the very top end. So, at the end of the day...my opinion for whatever it counts...i think the scoop did have some effect as there was solid improvement between the time of the scoop install and the carb rebuilt, however much unsure. Freshening up the carbs obviously helped. But bottom-line now, its really run wonderfully.

As an aside, the bike did amazing on the trip. I was really impressed. This was my first opportunity to really get some extended seat time and get a feel for the bike and get to know it. I couldn't have been more pleased. Me and a buddy went from Savannah, Georgia to New Orleans. 5 days, 5 states and a little over 1500 miles logged. He was a riding an '81 Yamaha XJ 550 Seca, and the Airhead kept pace step for step and beyond the whole trip.

Also, it became a running joke between us the number of people that approached us to ask about the Airhead. Every gas or food stop without fail at least 2 or 3 people came up to ask about the bike either find out what it was or to tell a story about one they knew. Practically every Harley guy wanted to know about it. My buddy said he was getting a complex that no one ever asked about his Seca, and that the only question he ever got was whether the Airhead was his.

Only had a couple of issues the whole trip. On the morning of the third day, when I was in the process of starting the bike it fired up cold, sputtered and died (which is not shocking), and when I went to try the starter again nothing happened, no click or anything. All lights were on and didn't dim when the button was pressed. No voltage drop across the battery when the button was pushed. I was thinking either the switch or the relay. Pulled the switch to try and jump it to see but didn't get anything. We rode in rain the entire previous day so thought maybe some corrosion on the relay, but didn't pull the tank to check at that time. It cranked great off the kicker. I'll get to tracing that down and testing the relay, etc. now.

The other issue showed up on the 4th day, the tach started to bounce around and eventually did the death spin that snaps the needle off and trashes the mechanism. I went ahead and disconnected both the tach and speedo cables as the speedo was starting to bounce as well. Wish I had known or thought to lube them on the list of everything else. I've already made contact with Terry/Wirespokes re: a rebuild.

But for that length of trip, again i'm quite pleased with this bike. Thanks to everyone for their advice and help. Much appreciated..
 
Good reason why all the bikes ought to have a back-up kick starter! I miss mine from my /6 and my /2 !!

That's debatable! I've never been so tired when I was learning to kick start my R69S...I really wish there was a button to push! It's been stated before...the kick start mechanisms after the /2 era weren't as robust as they were prior to 1970. They should really only be used as an emergency type of thing (ie backup as you say) as it's POSSIBLE to break something and really ruin your day.
 
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