Race bike idle and low end power.

[drgonzo]

I have a 1972 XS2 AHRMA production road racer. The engine is stock with a magneto ignition sold on ebay and build by another racer in California and made from chinese parts.
The bike has to be spun to about 2500 rpms to start so no kick or bump start. Only starter rollers will start it. The bike has foam unifilters and 1-1/2" OD pipes with short megaphones on it. It pulls hard on the top end and will out accelerate most bikes in the class. My problem is I can't get it to idle consistently below 3000 rpms and it falls flat on it's face under 4000. This has to be carb, exhaust or ignition related.
The carburetors are stock Solex/mikini 38mm units with 145 mains and 42 pilots.
I plan to add some baffles to my megaphones and see if this helps.
Any other ideas?

 

[jd750ace]

It was common for race bikes back in the day to skip the low end tuning because it has nothing to do with racing. Many AMA 4-cylinder bikes from the 70's would not run below 3500-4000 rpm because the bike never was running that slow anyway. Many tuners would deliberately run the "idle" rpm high to reduce the compression braking effect to promote stability on a closed throttle.
I imagine that many more who are more knowledgeable than me will chime in shortly. I just remember Kevin Cameron articles on the subject!

 

[drgonzo]

Yes I am aware of what you are saying but a stock based bike should pull from the basement and as I am not a pro rider it's nice to be able to overcome being in the wrong gear, etc.

 

[scabber]

If the motor is a stock 650 the carbs are where I would start. Find out what year the carbs are and start with the stock jetting. You are way too big on you jetts now.

 

[GreasyC]

Have you checked the timing or function of the advance unit already? If it isn't retarding the timing to the right point for idle that could cause problems. And you're sure the engine is all stock inside? Seems like a race engine would have at least porting and a cam suited to racing, which again could make running down low difficult, but I guess the class it was in prohibited that?

 

[jay760]

Is the engine a 1972 XS2 AHRMA production road racer or a standard xs650 engine? there seems to be some confusion over what you have, was the engine running before you put the ignition on,

how did you time the ignition as this is crank mounted so there are no timing marks,

I have recently built an engine similar to the original stated in this post, with racing cam, 750 kit and electrex world ignition http://www.electrexworld.co.uk/cgi-bin/ss000001.pl?page=search&SS=xs650&ACTION=Search&PR=-1&TB=A for the small extra cost not worth buying chinese, and 34mm dellorto's , mine doesn't run below 2500 revs, but does start on the kick, does your ignition even work below the revs you state? after seeing some of the chinese bikes, I wouldn't trust any thing that they make for themselves, they seem to be able to make parts for other manufacturers but when it comes to their own parts it is mostly rubbish,

 

[jd750ace]

Original bore and stroke are the only motor limits in that class, so a ton of things could have taken place. It's hard to make a race-bred engine work for the street, if that's what you are doing. If you want it to run that slow on the track, you're probably begging to get run over.

 

[drgonzo]

It's a swap meet bike. The engine is internally stock as far as I know. I have never had it apart. The rules are set up so that the carburetors are the limiting factor to performance.Stock 72 engine. Magneto ignition with fixed timing. The manufacturer of the magneto says his modified XS650 (750cc AHRMA SPORTSMAN) will idle at about 2000rpms. Mine will not. WHile I know my main jets are large my plugs look good and the bike pulls HARD with that jetting. Really the ignition is the new variable and I think it just doesnt put out well at lower rpms. I have the timing set at 35. Slightly lower might be worth a try.
It is a crank mounted ignition and I do have timing marks.

 

[jay760]

Do you still have the points, might be worth sticking a set in and just connecting a battery to them to see if there is any difference, a magneto should still work at less than 2000 revs, especially one that's derived from a road bike,

 

[mrriggs]

If it's the magneto kit I'm thinking of, then I don't think it came from a road bike. Pit bike maybe.

The power output of a magneto ignition is proportional to the RPM. The slower you spin it the less it puts out. My guess is that the ignition simply can't make enough juice to fire a high compression engine at low speeds.

Try closing up the spark plug gaps to see if it improves. Smaller gaps take less voltage. If the smaller gaps help then it's likely a weak spark issue.

 

[drgonzo]

OK, again this is not a high compression, modified race engine. It is a STOCK XS-2 engine that just runs really good on the top end. On a hunch I took the carbs off the racer and installed them on my XS1-B. With those carbs installed the bike only runs on the left cylinder or only intermittently fires on the right side UNTIL about 3500-4000rpms when it pulls like stink to redline. More work to be done. While no expert I have owned 4 of these bikes and successfully rebuilt numerous carburetors. Right now this one has me stumped.

 

[mrriggs]

OK, again this is not a high compression, modified race engine.

I just meant it was higher compression than the weed whacker that they got that magneto from.

 

[5twins]

Well, it does sound like a carb issue. Like maybe the pilot circuits are plugged. It may also be incorrect parts relating to the pilot circuits. Two types of pilots were used in the BS38 carbs. Each type requires a different float bowl. The difference is in how the air is delivered to the pilot. You must use the right pilot in the correct bowl or the idle circuit won't work correctly .....

 

[drgonzo]

I was thinking the same thing so I swapped bowls and the right side still is the problem. IDK?

 

[drgonzo]

I was thinking the same thing so I swapped bowls and the right side still is the problem. IDK? Maybe there is a restriction somewhere in the carb body.

I finally pulled a pilot jet from a VM mikuni and installed it and the bike started to fire some on that cylinder so i rode it around and it seemed to work itself out.
We shall see this weekend at Barber (fingers crossed).