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Plastic Main Jets?

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Guest

Guest
I took one of the bowls off of my 1978 GS750 to get an idea of the condition of the carburetors. First thing I noticed is that the carburetor was immaculate inside. Immediately after I realized that the main jet is plastic. Not the pilot jet just the main jet. The bike has an aftermarket exhaust so I assume someone rejetted the carbs and the plastic has a 120 stamped into it. Are these okay or should I go back to the brass. Also when re-jetting for an exhaust is it usually normal to move the needle as well? The bike runs super rich and I'm looking to get it running a little better. It currently gets about 28 mpg which I know it can do better. Lastly I plan to re-wrap the air filter with new foam but the bike has about a 1/4 spacer/gasket made of hard foam between the filter and the plate above it in the air box. Is this factory or should I leave it out?
 
I used to use plastic jets. They work fine, but that was also in pre-oxy fuel. Let people know were your jetted now, including needle number, pilot main jet sizes and someone will probably chime in with some good baseline settings to start from
S
 
Thought I'd revisit this thread rather than making a new one. I have since determined that the main is 120 and the pilot is a 15 but I am not sure of how to check the needle markings without taking them out. I also do no know how to take them out without disassembling the whole carb assembly. The bike has an aftermarket exhaust (Not sure which one) and a uni foam filter in the stock box. The other day I rode about 100 miles and the bike was running great. Just before I got home I decided to fill it up and choose to give the non-ethanol fuel a shot considering the plastic jets and all. The next few days and about 15 miles of just running around town (so very short trips) and on the last one the bike started running super rough, not sounding good, terrible throttle response and no power. I took the plugs out and they where very black. I've since only been about 20 miles and put another gallon of regular 87 but nothing has changed so far. Any thoughts?
 
If the bike was running great at any point it won't be jetting. They don't change sizes on the run. Maybe some dirt or other issue.
 
I think I would be checking your tank for rust or a failing tank sealer someone may have put in the tank. Check the petcock and rebuild the carburetors. Something broke loose and is blocking fuel flow. VM carburetors have a lot of tiny passages and I can tell you from experience they don't like rust or debris in the fuel system. You can tell absolutely nothing from the condition of the bowls, It's what you can't see that causes the problems. I have seen these carburetors look pristine and immediately discolor new carburetor dip within minutes of putting them in the solution. After a 24 hr. soak, you may not be able to see the carburetors below the surface of the dip.
 
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'78's were old school with points unless someone changed the ignition.
 
Yes yes check your points, then I would start adjusting the mixture and get it right. Black plugs, your running plenty rich.

V
 
I'm actually thinking it may be ignition related. Judging by the sound out the exhaust it sounds like it may be mis-firing or down 1 cylinder. What would be a good course of action on the points? Just clean them? Something like 200 miles ago or so I advanced the timing back to the factory spec as it was out by about 10 degrees and checked the points gap. I guess I'll re check to make sure nothing came loose. As for the carburetors my thinking is that any sort of blockage would make it run leaner rather than richer correct? It has an inline filter and the tank looks really nice inside. Last night I rode a little and pulled the number 1 and 4 plugs. 1 was actually wet with fuel and 4 looked as though it was starting to return to normal. I'm headed out the pull 2 and 3 now because it was too hot to do after my ride.
 
1/4 going out together is highly suggestive of dropping a coil. Be careful, as you could hydrolock a cylinder in this situation
 
1/4 don't seem to be going out, it seems to be all of them I just happened to only pull those plugs the other night. And I would really like to put the newer coils and electronic ignition but I'm stubborn and want the fix the old stuff first :D. When checking the plugs yesterday the threaded insert came out of one of the boots and stayed on the plug, so I'm going to take them all off and test the resistance and hopefully get the one back together. I've found a few threads on people assembling the resistors but if anyone has a better resource feel free to share

Edit: I got the boot apart, the resistor is long gone and the hole that the threaded insert goes into is egg shape and won't let me re assemble it. So I know I need a new one, but my question is my bike has 2 different length caps 2/3 are longer and 1/4 are shorter. Are these different lengths available in the aftermarket like NGK? I can't seem to find lengths listed anywhere
 
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I think I've finally figured out that it's a spark plug that went bad. I moved around a known good pug cap and the problem remained but the problem would move around with one spark plug. The question now is why did it go bad? It had the wrong plugs in it when I got it so I put in the correct B8ES plugs maybe 200 miles ago and one already went out. I think maybe due to the bad connection in the cap?
 
You are correct in thinking a bad spark plug is a rarity. But, if it wasn't firing correctly, it can foul pretty quickly
 
Yeah I have never had a plug fail on me before. I think it was due to the cap. I found the correct caps and ordered them and when they get here I'll replace the spark plugs as well. Until then I replaced the bad plug with one of my old ones and it runs great again. Although it is still slightly on the rich side. Does a 120 main 15 pilot jet sound like a normal combination for an aftermarket exhaust? I would really like to order new jets to replace the plastic ones
 
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