Choke out and it works great, choke off and it dies, down to 4000rpm again
#1
07-11-2011, 11:31 AM
Choke out and it works great, choke off and it dies, down to 4000rpm again
G'day all - Took off the carbs from my '83 GPPZ750 and put them to one side. Took a spare set I had off anī84GPZ750, cleaned, put together and back on the bike. Air box off. Took it for a run, idles well and great up to 4000rpm, then just starves for fuel. Choke out and it works great, choke off and it dies, down to 4000rpm again. Full throttle and it just dies until I back off.So I thought, I'd just put the original carbs back on (had been running fine, did the swap to see how the new set would run), and got exactly the same running issues. Only difference beteen before and after is that I 've left the airbox off while mucking around with carbs. Any socailly acceptable suggestions?
#2
07-21-2011, 05:01 PM
check fuel lines to be sure not pinched or collasping also fuel filter might need changing or if had filter off check arrow to be sure flow is going right way simple things or the most over looked good luck
#3
07-21-2011, 07:03 PM
Hmmm. Captain, I'm NOT trying to be a wise-*** here; just getting everyone's head on the same page. Your carb swap details are good. I'm not sure if the clues lead everyone to the same conclusions, and I want to clarify some assumptions and then make a deduction. Just to re-cap:
1. The bike WAS running fine. You did the swap to see how the spare set of carbs would work.
2. Both sets of carbs are identical? Same jet sizes, needle set-up, inlet diameter, etc?
3. Your rubber carb boots are intact? No cracks or tears? (Been on there since 1983?)
4. You have been using this bike, this year, running fine on fresh gas, before you did this carb swap? (I.e. no mouse nests or restrictions in the exhaust which would cause air flow issues?)
5. Again; (see #2) both sets of carbs are identical (same jets--primary and secondary, needle positions, blah blah) and the engine bogs at 4K on both sets? Need to verify WITH AND WITHOUT the airbox. Let's consider what happens when you "choke out and it works great." Choke-ON: you're decreasing the amount of air getting into the cylinders. This is significant because without the airbox, there is no air "restriction" effect (similar to what the choke does, so to speak) on the carbs. When you decrease the airflow percentage by pulling the choke, you (by necessity) increase the fuel percentage. You are exactly correct when you say the bike "just starves for fuel." The air-fuel mix becomes VERY important (even moreso) as you increase the amount of "flow" you put through the engine.
6. My deduction/guess, (again, assuming the carbs are identical) the issue is (primarily) airflow, and (consequently) the fuel mix is out of whack. Put the airbox back on for each test(yes; those older, between-the-frame airboxes are a PITA!)
Good luck. Let us know what you find.
(Gadz; my apologies. Bad form, Jeff.) Welcome to the Forums!!
1. The bike WAS running fine. You did the swap to see how the spare set of carbs would work.
2. Both sets of carbs are identical? Same jet sizes, needle set-up, inlet diameter, etc?
3. Your rubber carb boots are intact? No cracks or tears? (Been on there since 1983?)
4. You have been using this bike, this year, running fine on fresh gas, before you did this carb swap? (I.e. no mouse nests or restrictions in the exhaust which would cause air flow issues?)
5. Again; (see #2) both sets of carbs are identical (same jets--primary and secondary, needle positions, blah blah) and the engine bogs at 4K on both sets? Need to verify WITH AND WITHOUT the airbox. Let's consider what happens when you "choke out and it works great." Choke-ON: you're decreasing the amount of air getting into the cylinders. This is significant because without the airbox, there is no air "restriction" effect (similar to what the choke does, so to speak) on the carbs. When you decrease the airflow percentage by pulling the choke, you (by necessity) increase the fuel percentage. You are exactly correct when you say the bike "just starves for fuel." The air-fuel mix becomes VERY important (even moreso) as you increase the amount of "flow" you put through the engine.
6. My deduction/guess, (again, assuming the carbs are identical) the issue is (primarily) airflow, and (consequently) the fuel mix is out of whack. Put the airbox back on for each test(yes; those older, between-the-frame airboxes are a PITA!)
Good luck. Let us know what you find.
(Gadz; my apologies. Bad form, Jeff.) Welcome to the Forums!!
Last edited by jeffzx9; 07-21-2011 at 07:06 PM.
#6
07-24-2011, 09:03 AM
how best to get an air "restriction" effect" with sports filters?
Tks all for the input - I believe jeffzx9 has it with "there is no air "restriction" effect". Whilst mucking around with other stuff I put a rag over the uncovered carby intakes to keep bugs out - instant success, full power, no hassles. Took this off and put on the new sports filters and she then revved out to 6k, no problems. Airbox on would kill the problem. Any clues how best to get an air "restriction" effect" with sports filters? I was thinking of a gauze over the intakes and a bit of rag inside the new filters. Crude, I know ...
#7
07-24-2011, 05:41 PM
Might try K&N pod filters with a "pre-charger." Basically the pre-charger is an open cell foam "sock" to wrap around the outside of the filters. If you can't find a pre-charger for your application, get some 1/4" x 3" open cell foam and make your own. If this doesn't work, time to re-jet or put the stock airbox back on. Actually, with the variety of K&N, BMC, or UNI-filters available now for stock applications, the airbox isn't all bad. (Especially if it saves some extra work.)
Just don't put anything in the air path that could shred and get sucked in; very bad.
Glad you got it figured out!!!
Just don't put anything in the air path that could shred and get sucked in; very bad.
Glad you got it figured out!!!
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