Topic: Surging - Testing for Air Leaks

[Rosinante]

This bike surges at low speeds (and maybe a higher ones too).  Very noticeable. 

The boots between the carbs and the cylinder head are pretty important, I am told, and mine are likely 46 years old.  They feel more like plastic than rubber.  Engines create a fair bit of negative air and if they are not sealing completely, I think surging would be a result. 

So I wonder how I can test for leaks in those four boots.  I've heard of squirting stuff like WD40, carb cleaner, brake cleaner, etc.  If an air leak is present, it will suck these flammable liquids in and increase idle speed.  I have also heard of using propane or MAPP gas, which would also be sucked into the cylinders and increase combustion/idle speed.

I could also spooge some grease, like silicon grease or whatever, along all eight ends of these four boots.  This might temporarily seal whatever air leaks are present and make the surging to away temporarily.

Has anyone had success testing for air leaks using these methods, or another method?  If so, tell me about that.

Also, tell me if I am barking up the wrong tree here.

[fizzlebottom]

Treat the carb insulators like wear items. If they are 46 years old, or even 10 years old, they're ready to be replaced. Yes you can do your tests and such to see if leaky carb boots are the cause of your surging, but in all honesty you need to replace such old rubber anyways.

So, my advice is to get new carb insulators and then see if your surging is resolved. If it isn't, then move on to testing for vacuum leaks.

[HondaMan]

The boots for the K7/8 are only intermittently available (the good ones, anyway), but you might try this first:
those clamps that close the boots are pretty stretched out by now, right where the screw head is supposed to pull the joint closed. I've tested for bad boots by modifying the clamps like this (tedious, but cheap and works):
1. Pull off the clamp and note how far bent inward the metal face is around the screw's head. If a lot, go to Step 2.
2. With [nice] needle-nose pliers, flatten out the bowed-in inner face of the screw head's cradle.
3. With a grinder/Dremel/file/patience make a flat washer that also has flat sides to fit inside that 'cradle' to support the screw's head (I probably should find a picture of this imagery).
4. Put this fancy flat-sided washer into the clamp's screw cradle, then the screw, then put it back onto the bike. It will pull a hardened hose a little tighter now.

I used to do this a LOT from about 2002-2006 or so because we could only get intake hoses from EMGO, and they were genuine Chinko junk. I did it to my own 750 in 2001 (I think it was) and used them like that until the head removal (to fix a leak) in 2006. Just then, Honda was making some hoses again (a little bit smaller OD, but good rubber) and I replaced them, but they also shrank on their OD by about 2010, so the clamps closed completely but didn't grip tightly. Then I switched to using the Oetiker clamps instead, which solved things nicely with grip to spare. The Oetiker clamps aren't cheap, but they will seal them.

[HondaMan]

However:
surging on the F2/3 and K7/8 bikes with the PD41 and PD42 carbs also can come from missing bowl vent hoses. The K7/8 are especially sensitive to this issue: the hoses should reach from the bowl vents to well up underneath the seat, or else well back behind the airbox where the air is still. In a crosswind, without these little vent hoses well-shielded from busy air currents, the bowl levels move up & down erratically, causing the surging.

[Rosinante]

Ha!  I'm not always trolling for HM but I always hope he chimes in.  This time with enthusiasm.  Thank you!!!

And also hot shot fizzlebottom!  Two souls I recognize, though have never met.

I think the "insulators" which is what fizzlebottom and Honda calls them, appear available.  I have been warned against knockoffs, but it appears SS Honda may have them.  This is my plan, and while I have the carbs removed, it is past time I spend time on Carb Rebuild Island.

I hate it when you guys are right.  <smile>

[Rosinante]

Problem:  Everything is available from Honda except insulators for # 1, 2 and 4.  But....are these engines not symmetrical?  Would not the #3 insulator also be the #2 insulator?  If so, then why would they have different part numbers and prices?  (the prices are different by $0.98).

[PeWe]

If not a CB750 F2-3:
Try other carbs, K1-K6 version and the matching carb boots. Honda labeled.
Airbox another thing....

My K2 had my  K6 original carbs stamped 086A for a few years.
It got another set last summer stamped 064A that CB750 F 1975 had.
These work really fine. Choke only for start.

My K2 has K7/8 cam and CI +0.50mm K7/8 pistons. So almost an F...

[Rosinante]

Update:  I'm a little embarrassed to say this but I think the surging symptom I experienced yesterday was likely caused by a petcock that needed to be switched from ON to R.  Perhaps this should go into the "stupidest things we have done to or on a motorcycle" thread.

[HondaMan]

Update:  I'm a little embarrassed to say this but I think the surging symptom I experienced yesterday was likely caused by a petcock that needed to be switched from ON to R.  Perhaps this should go into the "stupidest things we have done to or on a motorcycle" thread.

Oh, boy...if you start that thread, I'll probably fill it up...

[fizzlebottom]

Haha! You aren't alone in making goofy mistakes like that. I'd join that boat a few times over.

[scottly]

[PeWe]

A pinched fuel hose that fills petcocks slower another reason for change main jets and adjust fuel levels without fixing the problem.

The stock style hose routed thru the carb rack beg to be pinched. A little is enough to get higher speed issues.

[Don R]

 I own a Suzuki that has a new replacement K&L petcock and a 5 gallon tank, it pretends to go on reserve when it only takes 2.5 gallons to fill it up. No way is that petcock not covered with gas. I'm baffled.

[newday777]

Problem:  Everything is available from Honda except insulators for # 1, 2 and 4.  But....are these engines not symmetrical?  Would not the #3 insulator also be the #2 insulator?  If so, then why would they have different part numbers and prices?  (the prices are different by $0.98).

Why? No idea.

JT Mark's on ebay has the #3 available. Not as cheap as from SS Honda......

[Rosinante]

Oh, boy...if you start that thread, I'll probably fill it up...

I did, yes, and you contributed:

http://forums.sohc4.net/index.php/topic,192433.msg2242571.html#msg2242571

[HondaMan]

Oh, boy...if you start that thread, I'll probably fill it up...

I did, yes, and you contributed:

http://forums.sohc4.net/index.php/topic,192433.msg2242571.html#msg2242571

Couldn't resist, had to add another sin to it...

[willbird]

Update:  I'm a little embarrassed to say this but I think the surging symptom I experienced yesterday was likely caused by a petcock that needed to be switched from ON to R.  Perhaps this should go into the "stupidest things we have done to or on a motorcycle" thread.

I still shudder to remember the feeling late 1970's early 1980's when one was forced to switch to reserve, doing so dumped the dregs of the fuel tank, water and rust included into your float bowls, your float needles were held open from gunk and the bowls overflowed, usually you limped to a gas station or limped home.

Bill

[Rosinante]

The only time I had float valve problems (a LOOONG time ago) was when something black and hard got into the fuel rail and held the valve open from above.  I never figured out how it got there, unless it was a piece of petrified fuel line.  MUCH larger than the petcock screen. 

One of the things that make me go "hmmmmm."

[willbird]

The only time I had float valve problems (a LOOONG time ago) was when something black and hard got into the fuel rail and held the valve open from above.  I never figured out how it got there, unless it was a piece of petrified fuel line.  MUCH larger than the petcock screen. 

One of the things that make me go "hmmmmm."

Well the 70's gas was pure gas and small local stations probably got a bit of water now and then, one really small one you were SURE to get water in your gas there. With pure gas a way to drain water out of the bottom of the tank would be good. Old school equipment and tractors had glass sediment bowls that were easily removed to clean.

[M 750K6]

I couldn't get my 750 to start after rebuild. I traced it to a plug of rubber blocking the outlet from the peacock. I had 'safely' stored my tank on a nice thick rubber / sealed foam pad. The weight of the tank was enough for the outlet to act like a hole punch, leaving a nice plug inside!   How I laughed....

[HondaMan]

I couldn't get my 750 to start after rebuild. I traced it to a plug of rubber blocking the outlet from the peacock. I had 'safely' stored my tank on a nice thick rubber / sealed foam pad. The weight of the tank was enough for the outlet to act like a hole punch, leaving a nice plug inside!   How I laughed....

That's one for the books!