If you like our site, please consider joining our club!
By joining you will help ensure that we can continue to provide this service
JOIN HERE!

Strange motor behavior at high speed

User avatar
Micha
Posts: 759
Joined: Fri Oct 12, 2007 2:24 am
Location: Israel
Has thanked: 10 times
Been thanked: 4 times

Re: Strange motor behavior at high speed

Post by Micha »

I came back to update: it seems that the problem was indeed a lack of fuel volume.
I filled the tank and went for a ride on the highway, leaving the in-line fuel filters in place.
At 100 km/h I stopped turning the throttle...
Thank you all for your responsiveness and willingness and help. Appreciate your time.
Michael Steinmann
R51/3 1952
Engine Nr. 529466

scottiesharpe
Posts: 135
Joined: Mon Apr 18, 2011 11:23 pm
Been thanked: 2 times

Re: Strange motor behavior at high speed

Post by scottiesharpe »

Too rich at full throttle? What is your altitude elevation? These bikes will do this at 4000+ elevation as the air thins out.
Scottie Sharpe, Proprietor
Scottie's Workshop, 3282 E Hwy 4, Murphys CA
Full Service Workshop for vintage and classic BMW Motorcycles http://blog.scottiesharpe.com

User avatar
Micha
Posts: 759
Joined: Fri Oct 12, 2007 2:24 am
Location: Israel
Has thanked: 10 times
Been thanked: 4 times

Re: Strange motor behavior at high speed

Post by Micha »

I'm near Tel Aviv, at sea level.
Michael Steinmann
R51/3 1952
Engine Nr. 529466

Seek
Posts: 164
Joined: Mon Jun 13, 2022 11:39 am
Been thanked: 12 times

Re: Strange motor behavior at high speed

Post by Seek »

It could still be too rich. I was also dissapointed with the performance of my R25. Changing the mainjet from 110 back to 105 gave me 5-10 km/h more topspeed

User avatar
Micha
Posts: 759
Joined: Fri Oct 12, 2007 2:24 am
Location: Israel
Has thanked: 10 times
Been thanked: 4 times

Re: Strange motor behavior at high speed

Post by Micha »

Thanks, I have no idea what main jets are there. Are they stamped? How can I know?
I use Bing's 1/24/159 and 1/24/160 bought new from Huggett ca. 10 years ago.

Another question: By changing these jets - is your intention to run the engine with a leaner mixture?
And if indeed so - isn't there a danger of overheating?
Michael Steinmann
R51/3 1952
Engine Nr. 529466

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

Re: Strange motor behavior at high speed

Post by schrader7032 »

Michael -

I'm sure Mark mentioned this as those carbs originally were put on the R50 or R50/2. Since your bike is also 500cc, likely it makes sense to use them. They are 24mm in bore diameter whereas the original was 22mm...I'm assuming you have some kind of reducer to neck down to the right diameter on the head.

The specs for these early carbs are shown on this page:

https://w6rec.com/wp-content/uploads/20 ... age1-3.jpg

This seems like a tough thing to figure out. I don't think you can use the settings called out on the R51/3 carb and maybe the settings for the R50/2 carb aren't right either. I'd probably start with the R50/2 settings and then evaluate all aspects of how the bike runs. I probably wouldn't worry about the main jet, as long as it was in the ballpark of the right size. Probably not very often you'll be running at wide-open-throttle...the main jet limits the top end speed. You're running around on the tapered needle over 95% of the time. So you would see how the bike accelerates from say 25mph to 50mph. If you grab throttle and it bogs down, you'll probably need to adjust the needle jet or the needle position. As for the idle side of things, getting the settings dialed in so the bike starts easily and doesn't bog down when taking off from a start.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

Seek
Posts: 164
Joined: Mon Jun 13, 2022 11:39 am
Been thanked: 12 times

Re: Strange motor behavior at high speed

Post by Seek »

Main jet is for full throttle and as far as I understand from the first post, that's exactly Micha's problem.
The best main jet is the one where you reach the highest speed. That's the way you determine the correct main jet. Then the mixture is just perfect, not too lean, not too rich. Usually four stroke engines have a bit of a range of usable jets, two strokes often have one jet outperforming clearly the other ones.

The main jet is of course in the carb body, at the underside. You first need to remove a cap (19 mm wrench, from memory). Then you unscrew the main jet with am 8 mm spanner. Often you take the needle yet with it, but that's no problem. It is marked on the jet what size it is. Get some extra's, look for a nice stretch of road at a quiet moment and start testing. One yet after another. When you don't race for minutes at top speed on end, you don't need to worry about to lean a mixture blowing a hole in your piston. Start at the current size and I would for now first go down in size and see if you have a difference in top speed.

A boxer engine in standard trim most often works perfectly well with the main jet size as indicated in the manual. So starting with that one is a good idea too.

After you determined the best main jet you can also check the needle setting. Mark half throttle (with tape and a pencil on your twistgrip) and go for a ride again. Look which setting gives you the highest speed when the marks on the twistgrip lines up.

Seek
Posts: 164
Joined: Mon Jun 13, 2022 11:39 am
Been thanked: 12 times

Re: Strange motor behavior at high speed

Post by Seek »

Oh BTW, do you have new pistons? Then you'd better first complete the run in period before you do any full throttle tests.

User avatar
Micha
Posts: 759
Joined: Fri Oct 12, 2007 2:24 am
Location: Israel
Has thanked: 10 times
Been thanked: 4 times

Re: Strange motor behavior at high speed

Post by Micha »

Thanks a lot guys.
I will jump into it as soon as possible. Lots of good info and ideas.

The pistons aren't new.
I don't have a reducing piece on the carb's flange, just a typical thick heat insulator (some pressed fabric, maybe micarta?).
Should I get them? Where are they available? Can't see them listed over at Salis.
Michael Steinmann
R51/3 1952
Engine Nr. 529466

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

Re: Strange motor behavior at high speed

Post by schrader7032 »

A reducer is not a standard BMW part...maybe Bing made something. It would be something that an owner would have to make to fit. I can't be certain on this, but given the carb numbers "1/22/xx" versus "1/24/xx", the 22 and 24 indicated a diameter of the carb throat. If that's the case, then going from the larger bore on your carbs to the smaller bore on the head, the air is going to hit an obstruction and will create flow issues for the engine.

If you're in contact with Mark Huggett, ask him what he thinks about this and see what he recommends.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

Post Reply