1976 GT250 Uneven exhaust

Mat · Posted: Mon Sep 26, 2011 9:13 pm Post subject: 1976 GT250 Uneven exhaust

I am having problems my 1976 Gt250. I bought this bike complete but not running. Went thru it this last winter. Found piston/cylinders worn so got cylinders bored out and got some new pistons off ebay. I cleaned out the carbs and put in new jets and seals. I took the baffles out of the exhaust to clean and while the left side was gunky and oily the right was dry and crusty which I though odd. When I had the jugs off I noticed right side crankcase had some nasty oil/old gas inside while left was fairly clean. Now I have bike running and while it runs fairly well I notice right side exhaust is dry while left is oily. Running left exhaust sound is crisper and louder while right is more muffled. I checked compression and to be expected on a rebuilt motor both cylinders have good compression and are even. I have only ridden bike about 20 miles checking it and trying different things. It is using oil and I even rebled oil pump and watched small air bubbles in oil lines to both cylinders move forward with engine running and me holding oil pump wide open so I believe both cylinders are getting oil. Right side exhaust(the dry side) was actually leaking some oil at pipe to cylinder connection which I tightened up but no oil out end of exhaust. Does anyone have any ideas? Maybe right side pipe is plugged before baffle? but when I had both pipes off I blew air from air hose thru them and they seemed ok. I put in new points and plugs of coarse and static timed it as carefully as I could. Bike looks good and I look forward to ridding it without worrying.

Tezza · Posted: Tue Sep 27, 2011 2:00 am Post subject:

Sounds like a carby mixture/balance problem. Start by mechanically balancing the carbs. Remove the intake boots and using a mirror wind the throttle open until the carb slides just disappear at the top of the carb body. they should both be even at the top. If not adjust the cables until both slides operate evenly. Make sure the ignition timing is correct. Now remove one spark lead and start the engine on one cylinder. Adjust the idle adjust ment until it runs at about 2000 rpm. Stop engine and do the same for the other cylinder. Now when you start the engine with both plug leads attached it will rev fairly high so carefully back out the idle screws the same amount until the engine idles normally. After warm up screw the mixture screws in until they bottom and the back out 1.5 turns. adjust slightly in or out to get the optimum rpm at idle. If the engine revs too high after you install both plug leads you can back out the idle adjustment a half turn on each before you start to keep the revs down a bit (helps if you are running in new pistons and rings)

Mat · Posted: Tue Sep 27, 2011 5:44 am Post subject:

Thanks for the info but I have done all that actually done it twice since I have had the carbs off looking at them a second time. I am thinking of pulling the right side exhaust off taking out the baffle and will try to run something thru it to see if I have any blockage as the more I think about it signs seem to point at that. I did blow air thru both exhaust when I had them off with air from a air tank but maybe it didn't have enough volume. I ran the bike briefly with the baffles out to see if there was a difference but I still had uneven sides.

Rizingson · Posted: Tue Sep 27, 2011 5:45 am Post subject:

Lots of possibilities here, did you replace any crank seals. Right side is wet seal (oil in crankcase using oil) Left side is dry seal. (running lean with exhaust dry and crusty). If you have any bad crank seals you will not be able to maintain a good consistent idle as well as bottom end performance, but performance above 4K or 5K may feel OK as you won't be able to detect any seal issues. Compression on a 2-stroke is only half the overhaul battle with the crankcase integrity being other half. As mentioned by Tezza first make sure you are properly synced at the carbs and then you might want to look at pressure/vacuum testing the crank case.

Mat · Posted: Tue Sep 27, 2011 6:11 am Post subject:

No I did not replace the crank seals hoping that they were ok and hoping to keep this project affordable. I did do a simple crank seal check after having the problem and thinking it could be bad seals. I checked the seals with rubber gloves covering up the intakes and exhaust and injecting low air pressure thru one glove. I detected no leaks between cylinders or side seals as far as I could tell with this simple test. Yes, I know the right side is the wet seal side but that is the side the exhaust is dry on while the left side exhaust is oily which I think is normal. Thanks for your input

MR.CHocko · Posted: Tue Sep 27, 2011 2:57 pm Post subject:

One thing about old two strokes, and I am saying this as a parts guy because I get lots of calls and emails about guys looking for two stroke exhausts is that they sometimes, will get gummed up. The oil from the exhaust will harden and form this rock like substance. You can blow air through it and air will pass, but when you go and ride the bike will perform terribly. I would first check to see if you have a bad seal and next I would check to see if you have any a blockage from old oil.

Mat · Posted: Tue Sep 27, 2011 9:12 pm Post subject:

I pulled the exhaust off again tonight to check for blockage. The right side pipe did have about 1/8" of scale in pipe coming off of cylinder but when I puled out baffles and put a small bright flashlight in pipe I can see that it is fairly clear. I did clean out about a hand full of scale. Both pipes are oily on inside coming off cylinder but baffle chambers on right side are dry and tan while left side is gunky oil. I will try to get them back on tomorrow and retry but don't think there will be much difference.

MR.CHocko · Posted: Wed Sep 28, 2011 1:52 am Post subject:

The right side should have oil residue. Try cleaning out more scale. There should not be any scale in the exhaust at all. What about the header pipes? They should be wet with oil residue. Are you running an air filter? Proper air flow is extremely important with these bikes. Also check to see if it is only firing on one cylinder.

Mat · Posted: Wed Sep 28, 2011 7:09 pm Post subject:

I didn't get a chance to put the pipes back on today and try the bike. Yes, there is oil residue in both of the header pipes coming of the cylinders but like I said the right side baffles are dry. Yes bike does have a air filter. Previous owner seemed to have replaced the filter and the foam looked clean and oiled so I did not mess with it. Bike starts right up and idles fine after it comes off choke and when warm starts right up and idles so no misfiring plug. When I took it out on road it seemed to run ok to me, not a lot of low end but I felt this was normal this being my first ride on a twin two stroke. I didn't push the RPMs as I just put in new pistons and rings.

MR.CHocko · Posted: Thu Sep 29, 2011 1:50 am Post subject:

If everything is firing, on two cylinders. I believe that your problem lies in the right baffle. But on the safe side check your ignition system, coils,etc.

Bikegeezer · Posted: Thu Sep 29, 2011 7:42 am Post subject:

dorT500 · Posted: Thu Sep 29, 2011 11:15 am Post subject:

Mat · Posted: Thu Sep 29, 2011 1:22 pm Post subject:

You are right Stu my simple test would only show extreme seal damage, it looks like I will have to do a better test. I reinstall the exhaust pipes and ran it without any change. I checked timing with timing light and it is dead on on both cylinders. I believe I did disassemble and clean the carbs well. I replace the main and pilot jets. I did not have the spec's on the float hight so I did not adjust but float arms for the two carburetors looked to be at the same hight to me. When I reopen the carbs I will check that using your spec's. When I first rebuilt the carbs I purchase and installed the wrong style pilot jet although it was the correct number for what should be in there. Bike would barely run so I did the "simple seal test" to rule out seal damage. On going thru the carbs again I found my mistake and corrected it. I do have fuel filters on the fuel lines going to carbs. Looks like I will be pulling the carbs and exhaust off again and do a more thorough seal test. I have a T20 that was to be my next project that I should do the test on anyhow. The T20 has been on the back burner. Thank you all for your input.

Bikegeezer · Posted: Sat Oct 01, 2011 6:32 am Post subject:

Mat · Posted: Sun Oct 02, 2011 4:44 pm Post subject:

OK, I did a proper crankcase pressure/vacuum test today. The right side was able to hold 6psi and 6" vacuum over 2 minutes while the left side slowly went down but still was able to hold about 5 1/2psi and 5 1/2" vacuum after a minute. My test equipment (rubber cork, rubber pipe cap could have been leaking a little bit so I would think my seals are good. I also checked the intake boots with soap/water mix and found no leaks. I guess I will go thru the carbs again and check that float height as well. Problem has got to be there somewhere. I will let you know what I find.