76 R90/6 - ideal octane/fuel choice

i'll probably bombard here with new threads for a little bit but

i've about depleted the full tank of gas that i got the bike with and need to fuel up. what should be my go-to when fueling up? premium? PO of the bike just told me he put "whatever the highest octane available was" in the bike.

FYI i've been riding the hell out of the thing so far and have been in love. rides like a dream compared to my last daily rider (96 sportster)....

Is the engine stock, i.e. not double plugged or base gasketed?

Highest octane, non-ethanol. Ride on!

swall said:

Is the engine stock, i.e. not double plugged or base gasketed?

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yep, stock.

I agree about running non ethanol fuel of the highest octane you can find. That is what I do with my R90S. I can get 93 octane non ethanol here in central Indiana. Keep the RPM's up and don't lug the engine. Back off if you hear pinging.
At some point I may consider adding leading race gas to my mix, but at $100 for a 5 gallon pail, I am not there yet.

My '76 had issues I believe were caused by ethanol. That said, if ethanol free isn't generally available I wouldn't worry about occasionally using ethanol fuel while out for a long ride.

My '76 R75/6 pinged even on 93 in hot weather but I think the R90 has slightly lower compression.

Your 76 R90 is a high compression (9.5:1) motor and when new required “96 octane or better” premium fuel with TEL (tetra ethyl lead). Both racing fuel and aviation fuel offer the chance to enhance today’s automotive fuel but you don’t need use more than a quart to a tank. If you decide to travel long distance you’ll wind up having to rely on pump gas anyway. Actually I would be surprised if your engine doesn’t suffer some detonation now. Adding base gaskets (de-tuning) and electronic ignition to help manage the ignition curve are the options available now to mitigate poor fuel quality issues. That or a gentle throttle to avoid ignition knock as noted earlier.

I think John is referring to the S engine.
The '76 R90/6 has a 9.0:1 ratio.
My R90/6 ran fine on premium with or without 10% ethanol.
I recently had the heads rebuilt by Beemershop, & installed the Siebenrock 1000cc kit without the base gaskets.
No problems so far with about 8000 miles on premium.

I used to run at 4000 to 4500 rpm on both of the Airheads I ride, until carbon buildup was found on the pistons and heads of the R100 while doing a top end reseal.
After being advised to run at 5000 rpm by several people that are far more knowledgeable about Airheads than I, there has been no issue with carbon buildup.

I owned a R60/5 that had 9.5:1 that would ping on hard acceleration and hot days.
Also had a '77 Bonneville purchased new in '78 that had 11.0:1 pistons & some other modifications that would PING on hard acceleration with the available at the time leaded premium.
I would add 102 octane at a 25-50% ratio to stop the pinging.

fuel

swall said:

I agree about running non ethanol fuel of the highest octane you can find. That is what I do with my R90S. I can get 93 octane non ethanol here in central Indiana. Keep the RPM's up and don't lug the engine. Back off if you hear pinging.
At some point I may consider adding leading race gas to my mix, but at $100 for a 5 gallon pail, I am not there yet.

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Recently discovered Ethanol free fuel near me and I started running that in my R90s this spring. Almost a complete transformation of the bike in terms of idle quality, low end response, etc. I've fallen in love with riding it again.

Not to mention, the ethanol enhanced fuel I was using last year, ate through one of the Dellorto float's.

concernedparent said:

i'll probably bombard here with new threads for a little bit but

i've about depleted the full tank of gas that i got the bike with and need to fuel up. what should be my go-to when fueling up? premium? PO of the bike just told me he put "whatever the highest octane available was" in the bike.

FYI i've been riding the hell out of the thing so far and have been in love. rides like a dream compared to my last daily rider (96 sportster)....

Click to expand...

My first BMW was a '76 R90/6 and in retrospect it was probably the best motorcycle I have ever owned.... never should have sold it.

In 1976 fuel degradation by ethanol wasn't an issue except in rare circumstances and leaded regular/premium gasoline was readily available. That was the world the R90/6 was designed to run in. Sad to say, it didn't last. One of the first things to suffer from the absence of (tetra-ethyl) lead in fuel were the valve guides and valve seats. Although they probably have been replaced by now you might want to check with the PO to make sure they have been replaced with parts that are designed for unleaded fuel.

I would highly recommend that you also consider joining the Airheads Beemer Club as well [www.airheads.org] a great source of information/parts/folly for all things airhead related. Combined with the MOA this will give you access to a wealth of experience.

There are a plethora of after-market items available to bring your /6 into the 21st Century and some of them are very worthwhile... some are junk. Don't get in a big rush to "modernize" your machine. The R90/6 was a good machine in its time and still is today. I put about 200K on mine with only two major modifications. First, I replaced the valve guides and seats to handle unleaded fuel. Second (and I do recommend this one), I fitted an electronic ignition system.

Welcome and congratulations! You have a great bike!

swall said:

I agree about running non ethanol fuel of the highest octane you can find.

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This fuel won't have high enough octane.

To boost octane requires an additive and now that tetraethyl lead and mtbe are illegal, the only octane booster available is ethanol.

10% is zero problem.

In my locale, 93 octane non ethanol fuel is available. Works fine in my stock R90S.

If you decide to use non ethanol, check out puregas.org.
They list all non ethanol stations in the country by state.
There is also an app for your phone that will list closest stations from your current location.

And get a fuel test kit to verify that the stuff being dispensed really is ethanol free.

Just fill up with premium ethanol free any time you can. Don't worry so much I just rode my '74 R90/6 to the RA rally filled up with what I could get made it back home and the motor ran like a clock. By the way got 40 mpg on the slab.