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Discussion Starter · #1 ·
OK here we go...
I introduced myself in the lobby a couple of days ago. I recently purchased a 1982 GPz750. The previous owner tells me it ran fine. Well, now it doesn't. It turns over fine and I have spark and gas comes out of the bowls. When stone cold and at full choke it will fire and run for a few seconds but it eventually dies and won't re-light. I'm thinking I need to pull the carbs and do some cleaning. I don't know the history of this bike so I don't know the last time it ran. Oh, I also checked the air filter and it was clean but the foam was dry. The gas smells good. What am I missing?
 

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Discussion Starter · #3 ·
Yeah, I didn't want to tear down carbs unless I had to. I bought it almost a month ago and have not even ridden it yet fer crissakes...
Thanks
 

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Vintage bike addict
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if it takes full choke to run even a few seconds. You MUST clean the carbs. Either take them to a professional or clean them at least twice yourself. Hunt for every tiny little passage and be certain they are clear. While you have the carbs out. Drain out the old gas from the tank and check inside for any sediment or rust. Use an aircompressor to blow through the fuel lines to be certain they too are clear. If you find any sediment in the fuel tank remove the fuel petcock and clean the sediment from the tank and clean the petcock as well. This done it should continue running and you diagnose other areas as needed.
 

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Discussion Starter · #5 ·
OK, will do. I'll report back with the results. I have some other problems I'll put in a separate post or three.
Thanks!
 

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Actually, before you go pulling carbs, it could be as simple as a fuel flow problem. Is the petcock on On or Prime? Is there enough fuel in there that it doesn't need to be on Reserve? Have you lifted the tank at any point, possibly pulling off or crimping the vacuum line going to the petcock?
 

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Discussion Starter · #7 ·
Actually, before you go pulling carbs, it could be as simple as a fuel flow problem. Is the petcock on On or Prime? Is there enough fuel in there that it doesn't need to be on Reserve? Have you lifted the tank at any point, possibly pulling off or crimping the vacuum line going to the petcock?
Good suggestion. I checked for fuel by draining a little out of the fuel bowls on the #1 and # 4carbs so I know it is getting fuel.
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Is there a seafoam strike??

Well, I'll take the plunge. If you really don't want to pull everything a bottle of seafoam in your tank might be enough to get the bike running. If it doesnt, it's 5 bucks, and you would have pulled the bike apart anyway.
 

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Discussion Starter · #9 ·
Update:
Success! I pulled the carbs and took them apart. This is what I found in the float bowls:

Yuck. The pilot jets were all clogged with the same crap. Cleaned 'em all, threw it back together and it roared to life.
Still a few issues. I noticed it had the stock jetting even though she's sporting a four-in-to-one exhaust. It seems to be running a little lean (backfiring and popping through the exhaust). How many sizes should I go up? It has 110 size main jets now.
Tomorrow I'll take it for a short ride. Then I'll start working on the other problems.
Thanks everyone, for your help.
JimmyD, I'm a believer in Seafoam but I don't think it would have helped much in this case.
Jim
 

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Navy Vet Search & Rescue
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If the 4-1 is the only mod and you still have factory airbox/filter you could probably use the mixture screw to get a little richer mixture. Worst case, I'd up the pilot jets one size and leave the mains alone. If just installing a 4-1, it doesn't normally require a jet change.
 

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The Cruising Gunsmith
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Still a few issues. I noticed it had the stock jetting even though she's sporting a four-in-to-one exhaust. It seems to be running a little lean (backfiring and popping through the exhaust). How many sizes should I go up? It has 110 size main jets now.
A lean main jet would only manifest itself by losing power at full throttle, high RPM operation.

If it's barking lean when you let off the gas, it's more likely the pilot jets.

You can get them in sets of four some places for about $9/set.

Check the one you have (see if it's stock). Then, you can go up one or two notches and see if it runs better.

FYI, back when I first put a really nicely tuned 2-1 on my 750 twin, I got best operation by going up one on the main jet (from 125 to 130) and increased the pilot from 40 to about 45.

YMMV, mine was a california bike that ran too lean to begin with.

The crappy MAC 2-1 header I have now runs best on the stock main jet (125) needs a rich pilot jet (about 47.5) and two notched rich on the needles.

Anyway, if your pilots are lean order some new ones. They are pretty cheap.

EDIT TO ADD: put some inline fuel filters on that thing.
 

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Yes, +1 to fuel filter. That rust came from somewhere. Or is it dirt?? :eek:
 

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Discussion Starter · #13 ·
update

Still has starting issues. I went out the next day and it was back to not starting. Took the carbs off again and went back through them. This time I took out the needle jets and the air screws too. I know, I should have done it that way the first time. Anyway, I am spending some time on the fuel enrichment circuit. I think I may have found one of my problems. In the bowl there is a hole in the side topwards the bottom that seems to feed the fuel enrichment circuit. See the picture below:

The orifice on the bottom right feeds the "well" on the side of the card where a brass tube slides into. Does that make any sense? As near as I can tell this is what feeds the enrichment circuit. Either that or I am way off. Anyway the holes were gummed up on all of them. I have all but one cleaned out. The last one is proving difficult. It's soaking in cleaner right now but I wonder if there is another way outside of an ultrasonic cleaner. Is there any danger is boiling it? BTW, these are stock Mikuni carbs.
Thanks everyone,
Sorry for the large pics. Is there a way to make them smaller?
 

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The Cruising Gunsmith
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I don't think boiling water will hurt them. Be careful about using very strong solvents as there may be plastic parts hidden somewhere.

Is there a jet down at the bottom of that tube in the bowl?

One trick I use is to get some Gumout carb spray and use the nozzle to shoot solvent into the various nooks and crannies. WEAR EYE PROTECTION.
 

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Discussion Starter · #15 ·
Sucess! Again!
I finally got all the crud out of the enrichment circuit. I blew air through all the passages and made sure that fuel could get from the bowl to the needle. I also went through and cleaned everything again. It started up pretty easily considering it was only about 35 degrees. Went out and rode it and it goes and stops just fine. I didn't get on it too much because 1) no license or registration 2) the road in front of my house is posted 30 mph and 3) I have not ridden a street bike in almost 20 years so I did not want to do anything stupid.
Stuff left to do:
Replace fork seals
Brakes
New front tire
Sync up the carbs
Fix turn signals
Fix ignition switch
ELR stripes
Thanks everyone! I'll post some pics when I put it back together.
 
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