I need a recommendation for mechanic who knows carberators very well | Page 2 | GTAMotorcycle.com

I need a recommendation for mechanic who knows carberators very well

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Brian P said:
Before doing ANYthing else, reset the valve clearances to specifications and check the compression again. You might be lucky, and the low compression is due to a tight valve clearance and nothing else. If proper compression cannot be achieved then here is what you are in for.

First, the cylinder head has to come off. I suspect that this requires the engine out of the bike (and life will be a whole lot easier doing the following work on the bench, even if the head can be done with the engine in-bike). But it's an air-cooled twin. Engine removal should be simple.

If the real problem is a burned valve, then at a minimum, the valve has to be replaced, the valve guide needs to be inspected for wear and repaired/replaced as necessary, the valve seat has to be re-cut and lapped with the new valve then cleaned up, and the valve stem seal has to be replaced. Then put the engine back together with a new head gasket. This is an air-cooled engine with two valves per cylinder ... this job is pretty close to as simple as it gets. But special tools and know-how are needed in order to repair the valve seats.

Once the engine is back together THEN look at the carbs, and it sounds like they need to be re-jetted.

Most common causes of burned valves:
Neglected (out of spec and ignored) valve clearance.
Overheating - air-cooled engines are susceptible to this.
Carbon build-up due to excessive short-trip driving (engine never warms up), excessive use of the choke, excessive rich mixture due to carb issues.
Carbon build-up because the engine is burning oil because of other problems (rings, valve stem seals, crankcase venting issues).
Carburetion set too lean near full-load. Lean idle and cruise won't hurt it (and helps to remove carbon build-up) but lean near full load is bad news.
Wrong oil, neglected oil changes, etc leading to valve stem and valve guide issues.
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Thanks Brian for the very detailed post. I have all the tools ready and will be doing a valve adjustment this coming week. I will take pictures once the engine cover is off the bike (gs500's are easy to work on while the engine stays in the bike). I am not too sure what I should be looking at, however I feel certain that my bike is suffering from burnt valves. My valves haven't been adjusted in nearly 20k or more and power has been dropping in my gradually over the past year and a half (top speed of 110-125km/h with no windscreen). I have seriously neglected the needs of my motorcycle and I am being punished :p.

As honduhmatic says, if I can find an engine on the cheap that is a resort I may have to take. However I feel positive that I will not need to resort to that.

A question? If I have burnt valves does the entire top end of the bike need to be replaced or can I replace the valves individually?
 
How did you come to the conclusion that you have a burned valve? Only the loss in power you are describing?

junk301 said:
A question? If I have burnt valves does the entire top end of the bike need to be replaced or can I replace the valves individually?
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Brian has basically answered this above - unless there is very severe damage to the cylinder head (e.g. badly warped, bent valves, etc) it is probably possible to do what he described. If you are asking if you can expect to replace less than all four valves, I'd say it is unlikely - if one is burned, the rest are probably damaged in some way as well.
 
So back to post #11, snapped main fuel mix screw? did it snap all by it self or did it have help? my reason for asking is if somebody that didnt really know what he was doing leaned out the engine and it ate a valve, you'll need to follow Brian's steps and see what up with the heads, get the valves in and reseated, then be sure your carbs get put right before you put any miles on the bike because you'll eat your valves again . And that would suck.
 
TwistedKestrel said:
How did you come to the conclusion that you have a burned valve? Only the loss in power you are describing?



Brian has basically answered this above - unless there is very severe damage to the cylinder head (e.g. badly warped, bent valves, etc) it is probably possible to do what he described. If you are asking if you can expect to replace less than all four valves, I'd say it is unlikely - if one is burned, the rest are probably damaged in some way as well.
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I'll find out what the damage is and post back beginning of next week. thanks for the response.
 
crankcall said:
So back to post #11, snapped main fuel mix screw? did it snap all by it self or did it have help? my reason for asking is if somebody that didnt really know what he was doing leaned out the engine and it ate a valve, you'll need to follow Brian's steps and see what up with the heads, get the valves in and reseated, then be sure your carbs get put right before you put any miles on the bike because you'll eat your valves again . And that would suck.
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I took my bike to a shop(what is it now? 2 summers ago?) Anyways I felt that my bike was losing power on the freeway. They took the bike, and they did a few things. I asked them to cut my pipe in half due to a hole in the top half, and then they rejetted the carbs. They couldn't get the bike to run properly and after tinkering with the bike for 4-5 hours, they finally gave it back to me (they were adjusting fuel mixture because it kept running too lean or too rich). Not sure what they exactly did, but it the end it ran a little better. Power was still not what it used to be. Also, there was an idle issue and the bike would never idle anymore (I beleive this was before I brought the bike in). So this past summer, my bike lost even more power. No acceleration past 90km/h. Brought it to a different shop. They opened up the carbs and they told me one of the fuel mixture screws was snapped inside the carb. After getting a new one, problem wasn't fixed, their only answer for me was that I needed new carbs (the alarm went off in my head). They couldn't figure out what it was and gave it back to me (well that's a reputable mechanic). Not one mechanic before asking on these forums(gstwins as well), even brought up a possible valve issue. I'm lucky I didn't throw that much money at those mechanics. Well here I am now. I rented myself a garage for the next month and a half and I am finally going to have a go at this myself.
 
The way I see it, you lose nothing (and gain experience) by pulling the engine out and removing the head. It'd be a great learning experience.

To go all the way down to 35 PSI compression, I'd be worried that there's piston ring damage as well as valve damage.

Good luck!
 
Okay. I took off the valve cover and inspected the valve clearances. Both intake valves are not in spec (0.03-0.09). I did not get a chance to take out the shims and order new ones yet. Here's my question. As I was hand cranking the engine, there were times during the revolution where I felt more tension than other times. I was wondering what the reason for this is. For example, as I was cranking the engine slowly, there was a constant normal tension, then all of a sudden it was loose, as if something was stuck and just got free.
 
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what shops did you take your bike to? this is 101 trouble shooting. 33k's check the valve clearance!
 
Update! I checked the valve clearances with my feeler gauges and both the left and right exhaust valves didn't fit my feeler gauges from 0.02-0.09..however both intakes were fine (they both fit from 0.02-0.05/0.06). However when trying to remove both exhaust valve shims, I was unable to because the shim buckets were too tight to move around. Does anyone know how I can take out the shims without having the valve buckets loose so I can use a flathead to pry out the shim... Any help is appreciated.
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