Topic: Can you stand another carb question?

[gpzkat]

My 73 is stock except for MAC 4:2 slip ons.  Carbs have been ultrasonically cleaned, floats initially set by measurement.  Used my mercury carb sync and adjusted the mixture, and the bike ran strong up to redline. But it was cold blooded, requiring a couple minutes and choke fiddling to warm up, and would spew fuel out the overflows when taking off the center stand.  I got a clear tube adapter and set to 3-4mm below the bowl mating surface, did on workbench with the carbs level. It was definitely high.

Started it up and wow, right away the bike needed very little choke and ran great in the garage at least.  No more overflow problems either.  I did the carb sync again and checked plugs, they're perfect.  Took it out, and up to 5500 RPM or so it's fantastic, but then it just bogs and feels like fuel starvation.  I didn't touch anything else.  Valves are spot on, engine is tight and has even and good 150 compression on all 4 with a recent top end refresh.  The only thing I did was set the floats.  On the odd chance it was coincidence with something ignition related, I checked the advance mechanism, no problem there.

Taking carbs off again is probably in my future, but I thought I'd consult you guys on what to look for, I'm kind of stumped.  Re-measure and possibly go a bit higher with float levels? That's all I can think of. Seems extremely sensitive.

[TwoTired]

If it's fuel starvation, the plug should look really really clean of deposits if you stop the motor while presenting the problem.

73 500 or 750?
Stock air box and clean filter?

There is cleaning with hope and cleaning with verification.  Simple bath is not verification by whatever method.

 Emulsion tubes all clean?  Do the jets have the K symbol on them? What size are the jets?
 Jet needle position?

[pjlogue]

Gas line tubing routing is very important.  I had a dickens of a time when I first restored my 750.  I had put a fuel filter in line and in order to do that I ran the fuel tubing around quite a bit.  I found that in hot weather the fuel filter seemed to have vapor in it and when this happened my bike would bog down under load (going up hill at highway speed)  I got rid of the filter and made sure the fuel line did not have any "n" bends in the line where vapor/air could get trapped.  The straightest run from the fuel spigot to the carb and all down hill is ideal.  A "u" bend is OK.

-P.

[gpzkat]

'73 CB750, sorry.  New paper air filter, stock air box. Carbs cleaned with ultrasonic but also checked all the passages, Compressed air and carb cleaner. etc. They are spotless and were working fine.  All of the jets and emulsion tubes are perfectly clear, they got a thorough cleaning and I double checked when setting the float last week.

I'm certain that the needle position and jets are as original as I checked per the year and engine number, but I didn't save that data so I can't recall the numbers.  I double checked the sizes and positions to ensure they were per stock and correct, but didn't take pics. This CB had never been apart and was wholly original throughout so I kept as-is after verifying it was stock setup.

I've done a plug chop at the highest RPM it'll hold, and they are all quite clean.

If it's fuel starvation, the plug should look really really clean of deposits if you stop the motor while presenting the problem.

73 500 or 750?
Stock air box and clean filter?

There is cleaning with hope and cleaning with verification.  Simple bath is not verification by whatever method.

 Emulsion tubes all clean?  Do the jets have the K symbol on them? What size are the jets?
 Jet needle position?

[gpzkat]

Thanks. Mine are going through the proper holes in the carb mount housing and are set to fit the petcock without much slack. Is a bit of a pain to get them on/off but no kinks due to extra lines, or binding. And I didn't change anything there, it's all as it was when it was running up to redline before.

Gas line tubing routing is very important.  I had a dickens of a time when I first restored my 750.  I had put a fuel filter in line and in order to do that I ran the fuel tubing around quite a bit.  I found that in hot weather the fuel filter seemed to have vapor in it and when this happened my bike would bog down under load (going up hill at highway speed)  I got rid of the filter and made sure the fuel line did not have any "n" bends in the line where vapor/air could get trapped.  The straightest run from the fuel spigot to the carb and all down hill is ideal.  A "u" bend is OK.

-P.

[gpzkat]

One thing just came to me, my '79 GS1000 stopped running well around the same time. With nothing changed to the bike it suddenly won't take any throttle and has to run on full choke.  It was running great after a top end/carb rebuild similar to the Honda, with several hundred trouble free miles, then pffft...  I am wondering if the local gas station is the culprit?  It would be odd to have such different outcomes but maybe due to amount of fuel I put in each.  I think some 'Heet' is in order, it's a cheap test.

[newday777]

One thing just came to me, my '79 GS1000 stopped running well around the same time. With nothing changed to the bike it suddenly won't take any throttle and has to run on full choke.  It was running great after a top end/carb rebuild similar to the Honda, with several hundred trouble free miles, then pffft...  I am wondering if the local gas station is the culprit?  It would be odd to have such different outcomes but maybe due to amount of fuel I put in each.  I think some 'Heet' is in order, it's a cheap test.

Bad gas or rust in the gas tank flakes getting in the carbs.
No added fuel filters?

[goodtryer]

Is there any sediment in the carb bowls? That’s what I’ve seen every time I’ve used a rusted tank.

Maybe you could try connecting a temporary tank to see if the problem goes away.

[gpzkat]

Bad gas. Just put in Heet, ran it about 10 miles and it will now hit redline with a tiny bit of a stumble at around 8K. I think it's just working itself out.  I will mix in some (hopefully) good fuel later.  So frustrating because I took the carbs off my '79 GS1000 due to an off idle stumble. I'll add the Heet to it and some good fuel and hope for the best.

I previously lined the tank with POR15 which turned out great, had a couple of pin holes on the top front seam that are now sealed.

I know people like pictures, here's my current slew of bikes:

1973 CB750 resurrection - long neglected non-runner, all gone through.



1979 Suzuki GS1000 - also a long neglected non-runner, all gone through.



1982 Suzuki GS1100E Mr Turbo - ex Bonneville racer, built by Don Vesco's shop in '82, 184MPH!

Bad gas or rust in the gas tank flakes getting in the carbs.
No added fuel filters?

[ekpent]

 I've got your twins with the orange 73 750 and 79 GS1000 but no silver 1100 in the stable yet  Been riding my GS1000 quite a lot lately,almost 1,000 miles since I busted it back out awhile ago.
   Never bought any bad gas that I know of. What brand did you in ?

[gpzkat]

Cool. I also have a '66 Norton N15CS I'm doing a ground-up restoration on.

It was from Shell. Hoping for similar result on the GS.  The coincidence was just too strong to have two bikes start to run like garbage with no changes and both being gone through top to bottom. The light bulb lit up for once 

I've got your twins with the orange 73 750 and 79 GS1000 but no silver 1100 in the stable yet  Been riding my GS1000 quite a lot lately,almost 1,000 miles since I busted it back out awhile ago.
   Never bought any bad gas that I know of. What brand did you in ?

[newday777]

Good to hear you found it out. Bad on Shell station though.
I looked at the GS in 79 and I should have forked out for the Wes Cooley version....... But I kept on riding my K6.

[gpzkat]

Yeah that Cooley is nice, many make replicas. I even had the oil cooler but sold it, times are tight and it fetched good money. I'm not racing and the bike doesn't really need it in normal street use, though it does look the business.

I also lusted after the KZ1000 ELR but bought the KZ1000J new, as there weren't any ELRs around.  It was black too. The Kawi was a nice bike but the Suzuki is better handling and not an oil burner. They are damn near indestructible. 

The GS is running well now too, still a bit stumbly but will take throttle so I'll top it up from another station and I'm sure it'll sort it out.

At one point I had these two, both '82s, the GPz was punched out to 1166 and ran the original FI.  The Katana had pods, Kerker, and Fox piggyback, polished swingarm and powdercoated frame. Wish I'd kept them, I had about $3K into them and sold to a Japanese exporter, he was very very happy.

Turning this into a 'bikes I've owned thread' but oh well, carbs are boring.



Oh and here's the CB as I bought it last year:

Good to hear you found it out. Bad on Shell station though.
I looked at the GS in 79 and I should have forked out for the Wes Cooley version....... But I kept on riding my K6.

[PeWe]

Rusty air vent in fuel cap stopped my bike after a short ride a few years ago.
Non Honda fuel cap gasket can be wider and also block the vent hole.

[newday777]

You did a great job on the CB.
I noticed the key had been mounted on the right when you got it.