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Tips

SU carbs on BJ7

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JKent

JKent

Freshman Member
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I recently got my BJ7 going again,and find that the carbs are running way too rich. Worked fine while breaking in the cam, at 2000-3500 RPM, but isn't doing well otherwise. Electronic fuel pump is putting out 4 psi, so I stopped it down to about 2 psi with a pressure regulator, still runs rich, even with the jets cranked all the way up, level with the bridge. No evidence of sunk float or stuck needle valve (no overflow), but runs okay if fuel is trickled into the bowls by intermittently running the electric pump. Float levels set per the manual (nylon floats, 1/8" clearance). Any suggestions?
 
JKent said:
I recently got my BJ7 going again,and find that the carbs are running way too rich. Worked fine while breaking in the cam, at 2000-3500 RPM, but isn't doing well otherwise. Electronic fuel pump is putting out 4 psi, so I stopped it down to about 2 psi with a pressure regulator, still runs rich, even with the jets cranked all the way up, level with the bridge. No evidence of sunk float or stuck needle valve (no overflow), but runs okay if fuel is trickled into the bowls by intermittently running the electric pump. Float levels set per the manual (nylon floats, 1/8" clearance). Any suggestions?
Click to expand...
I purchased a rebuild kit several years ago with mismatched jets and needles. The jets holes were too large in diameter and the needles were to narrow. While you're checking this out you might as well verify that the needle is in the correct position at the base of the carb piston.
Patrick
 
Yes, use the mixture adjusting screws. Decreasing the fuel flow/pressure and changing the float bowl level is not the way to change the mixture.
 
Pull the vacuum dashpots off and turn the key on. With the pump running--it should stop when the float bowls are full--note the level of the fuel in the jets. With the jets all the way up I would expect it to be about 1/8" below the opening of the jet. Much different than this and the floats are not adjusted correctly.

Like Patrick said, check the needle position in the vacuum piston--this can make a big difference in mixture. If the needle has a square shoulder, the shoulder should be level with the bottom of the piston. If the needle has a groove, the groove should be just inside the piston; i.e. bottom of groove should be level with bottom of piston.

If the pump continues to click more than once every 10-15 seconds or so you're flooding--you might see fuel flowing out the top of the jet. Also, take a look at the jet opening--if it's been oblonged or widened by the needle you'll need new jets and needles.
 
Pulled the domes off, and the needles were set right at the shoulder,although they seem a bit worn. Re-checked float level (with Grose valves) and pressurized the system. Got some weeping of fuel out of the jets, so presumably the floats and valves aren't working correctly. Will order needles (jets are new)and floats today, and see if that does the trick. Thanks for the tips.
 
I wouldn't order anything until you determine the actual problem. From what I hear, the replacement floats ain't the greatest and if your originals aren't leaking you're better off with them. I think at this point you need to pull the carbs and disassemble them to see what's wrong.

Think the BJ7 uses HD carbs, no? If so, the diaphragms could be damaged, even torn. That would cause fuel to leak past the jets. You didn't say where the jets are 'weeping,' but if it's around the outside of the jet it's probably the diaphragm(s). Are both carbs leaking?

Needles shouldn't wear--they should never come in contact with the jets.
 
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Pulled the domes off, and the needles were set right at the shoulder,although they seem a bit worn.

If this is the case then it is possible that the jet bearing is not properly aligned or the needle was inadvertently bent while handling.

Re-checked float level (with Grose valves) and pressurized the system. Got some weeping of fuel.
Do the carbs leak fuel out the bottom???
How old are your "Grose Valves" ???---Keoke
 
Healeysince59 said:
Highly recommend trashing the Grose jets and going to Viton tipped needles.

Marv J
Click to expand...
DITTO....I used the Grosae Jets too for a long period of time and then got rid of them and my problems. Think of the vibration of your engine when it's running...the jets vibrate and the gas seeps past the "ball & seat".

The Grose Jets allow you to think you fixed the problem, but in reality, you may have more than one problem!!
Patrick
 
stevebn2bj7 said:
Bob, the BJ7 uses HS-6 carbs. Much different than the HD carbs and in my opinion much harder to tune.
Click to expand...

Ah ... learn something every day. I presume it's possible for fuel to leak past the outside of the jets with this type as well?
 
Yes it is. There is a rubber tube that goes from the float bowl to the needle and seat. You adjust the idle by moving the seat up and down.
 
Grose valves are new, and the forward carb is leaking more than the rear, with fuel coming out of the center of the jets. No leakage out of the bottom of the carb or float bowl. I still have the float valves that came with the rebuild kit, so if new floats don't fix it, I'll try the original style float valves. Not sure mis-matched jets and needles are at fault, although once the leaking through the jets is resolved, that is something to look at.
 
Good day J Kent. I too have a BJ7 and rebuilt the carbs myself. Try completing each step in the manual, I found it easier to do both at the same time. Pay particular attention to where they do the drop test. I found one was sticking about half way down. Don't cheat, follow the manual and take your time. It'll pay off in the long run.
Good luck.
 
Hi Kent.
Reportedly, the new Grose Valves are coated with a presevative that is not dissolved by the gasoline. It is suggested that you soak the valves in Acetone and flush thoroughly Pror to use. Fwiw--Keoke
 
Actually had that problem after the rebuild. Re-centered the jets, and now the pistons drop with a satisfying clunk.
 
Tried new floats with both Grose jets, and the original style needle valves from the re-build kit. Both carbs still flood from the jets, suggesting that the floats don't have enough buoyancy to stop off 3 psi of fuel pressure, or are binding somehow. Do the Viton tip needle valves make that much difference? Is it worth sending them out to a professional?
 
Do the Viton tip needle valves make that much difference?

Not in my opinion just another plastic item to take a set and
and quit functioning.

Is it worth sending them out to a professional? Certainly, However, it just may be that the kits you purchased inadvertently had the wrong sized jets in them. It might be worthwhile to order a new set of jets from Joe Cutro as an interim solution.-Fwiw-Keoke- :confuse:
 
Could be the wrong jets, but I don't understand how that would contribute to flooding with the domes and pistons removed, the engine off, and the fuel system pressurized. I've blown out both the needle valves and the Grose-jets, and verified that either will shut off with the the bowl tops held inverted, and the weight of the floats holding them closed. That done with breath vice pressurized fuel, BTW.
 
JKent said:
Could be the wrong jets, but I don't understand how that would contribute to flooding with the domes and pistons removed, the engine off, and the fuel system pressurized. I've blown out both the needle valves and the Grose-jets, and verified that either will shut off with the the bowl tops held inverted, and the weight of the floats holding them closed. That done with breath vice pressurized fuel, BTW.
Click to expand...
Even if you have the wrong jets they should not let gas get past them with the pump "on"(engine not running) and the pistons removed from the carbs. If gas is getting past the jets you have other problems such as damaged jet diaphram. I can take my pistons out on my BJ8 and with the pump on(engine not running), no gas gets by them. Possible sealing issue with the jets/carb surfaces assuming that the floats and needle/seats are in good working order. Pump pressure shouldn't be more than 3.5 psi, but the carbs will take more than that, but NOT too much more.
Patrick
 
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