VHB 29 Help...Update.

[Tom H]

My '70 750 has been having carb trouble. It's running rich. It uses 3.5-3.8 gallons for 120 miles of my mainly freeway drive. My Eldo and EV use about 2.8-3.0 for the same run.

It has been running like this for quite some time, like years, this did not happen overnight. It has become worse recently.

 Recently I read a post that mentioned an issue at full throttle that the bike would loose power/run flat, like it was too rich. The choke slide was a possible solution. I changed the choke slide on both carbs with new slides and it helped with the full throttle flat spot I had, but still runs rich.

It idles fine and runs fine, but it runs rich throughout the throttle range. Until recently, I did use the idle stops to make the bike idle. But now to get it to idle down, I have to back out the idle stops all the way. It will not stall with the stops fully screwed out.

One thing I noticed while on a ride today. With the throttle closed it idled fine as usual with the idle stops backed out. But when I twisted the throttle just a tiny bit, it acted like it wanted to stall. Little more throttle and it pick up as it should. Even went back to normal idle with throttle off. Is there a clue there?

The timing is right, point adjusted, good Bosh blue coil, plugs are good as well as the wires, valves adjusted. It runs the same with or without the air cleaners installed. It does have the small cigar mufflers. BTW: tried the MAC's from my Eldo to see if that would help the bike run better a few years ago, it didn't see to like them, maybe because of no crossover??

What I have done so far:
Changed the choke slides.

Changed the O ring on the choke jet tube.

Made sure the ball rattles in the pump and passages are clear and springs are working.

Made sure all jets were clear and the correct size per the book.

Floats work freely and the float needle does stop the fuel.

Made sure all 4 cables have plenty of slack and are not holding anything open.

Moved the main needle down one notch a few weeks ago. Didn't change anything, moved it back to center notch per book last week.

Adjusted mix needles per book, 1.5 LH 2.0 RH is I remember the RH/LH right.

I have cleaned and adjusted the carbs on both bikes over the years. So it's not my first time around a carb. But I'm not the best at carb tuning.

So what would cause it to run rich? Worn throttle slide, worn carb body, worn needle???

I had the same thing happen to my Eldo where it had the same symptoms. Fixed it with a spare lower mileage set of cabs that I had in my stash. So now I have two sets that I need to fix.

I could switch the good working 850 carbs to the 750 and vice versa, but I think that will just tell me the 850 carbs are good, but not WHY. Depending on what answers I get, I may swap the throttle slide between them and see what happens. Until I learns a bit more as to the cause of the problem, I'm a little hesitant to move parts between bikes.

Any help with this??
Tom

[fotoguzzi]

swapping the throttle slides from the 850 carbs would be a good test..

I forgot most of what I thought I knew about DelOrto's long ago.. but how many miles on these carbs?

[SED]

Hi Tom,

I've not been into VHB carbs so probably not the best resource, but the fact that you can turn the idle stop screws (not mixture screws) all the way down sounds really weird to me. 

Did you set the idle mixture for best idle running on one cylinder?  (I'd bet the original 1.5 turns out is different do to wear, corrosion or deposits.)

If the throttle cables are too tight they would hold the slide open, but it sounds like you checked this. 

If there is an airleak then that would allow some air in even though the slide is all the way closed.  So airleak between the carb and manifold or the manifold and head?  OR an air leak around the slides.  Your new slides should have made a difference unless it is the carb body that is worn. 

How is the engine compression?  My perception is that low compression engines idle easier with less fuel and air and it might explain poor economy.

If the ignition timing is advancing at idle (worn advance springs?) perhaps it would not idle down and give too little advance while driving decreasing your mpg.

As for poor economy - floats getting progressively saturated with fuel?  Also the needles and atomizers will wear letting in more fuel than designed. 

Let us know what you find.

[Tom H]

Miles on the 750 carbs...Good question. Not sure which set I put on this bike.  I put this bike back together again after a number of years down time. Did the Guilardoni (SP) set and checked the bearings at that time.

Ever since I've had this bike, it's always been sluggish compared to the 850. Now I'm trying to make it work it's best.

Advance springs. Good question. I've had the advance weights get stuck before on both bikes. I will check that again to make sure they move right. BUT, the 850 had the same problem as the 750 has now, and the newer carbs fixed it.

EDIT: the advance unit seems to work fine when I when time it with a timing light.

Floats Hummm. These are the hollow plastic style. I don't feel/hear fuel in them.

The main needle I had moved down one notch with no change.

Air leak at the manifold to head, changed the gaskets when I got the bike running as above. Bolts are tight.

Air leak at the throttle slide is the question? I would have thought that would make it run lean, not rich?? They do have scars on the head side of the slide. The air cleaner side looks fine.

Thanks so far!!
Tom

[wirespokes]

I'm not super familiar with these carbs, though I have worked on the round slide 38 pumpers. If running the idle screw all the way out doesn't stall the engine, I'd guess there's an air leak. Maybe a manifold gasket? The top cover o-ring?

For running rich, there are several things to consider:

The chokes could be open a bit and dribbling fuel all the time.

The float level is too high. If so, it takes less "suck" (more fuel can be drawn) to pull the fuel up into the engine. I don't know what the level should be or how it can be checked - but guess by removing the float bowl drain plug and catching the fuel you could measure how much is in there. With the Bing carbs, it's easy - just remove the float bowl and measure down from the top.

The needle jet and jet needle can be worn. Take a look at the needle for shiny polished areas. It's possible the jet is worn out from the needle rubbing, and even though the needle was dropped, the jet is so oversize it didn't make much of a difference.

Also, weigh the floats and make sure they're the correct ones for this bike.

[Marcelli]

Hi Tom

According to Greg Bender:

Carburettor   Float height, millimeters
PHBG   16 +/- 0.5
PHBL   24 +/- 0.5
PHBH   24 +/- 0.5
PHBE   18 +/- 0.5
PHF   18 +/- 0.5
PHM   18 +/- 0.5
VHB/Z/T   24 +/- 0.5
PHSB*   11 +/- 0.5
VHSA*   11 +/- 0.5
VHSB*   11 +/- 0.5
* PHSB/VHSA/VHSB one piece float assembly.

In your case: VHB/Z/T   24 +/- 0.5

It sure did help me to adjust the float height.
My Ambo-71 were running rich too, until I adjusted the float height

Good luck!

//Marcelli

[Rich A]

I may have missed it, but did you check to make sure that the enricheners are sealing properly?

Rich A

[Tom H]

I swapped the slides from my good running 850 to the 750 and it made a difference. The idle stop screw now needs to be screwed in or the bike will stall. It is still running rich. I'm going to scrounge through my stash of parts and see if I can find a pair of good slides and a better set of slide needles.

I'll check my float height and see if that will help the richness once I find a good set of slides and see how much of a difference they make. Solve one problem at a time, or you don't know which solved the problem.

I did replace the choke slides previously. I read a post about an issue of too rich at wide open where the bike would bog down unless you let off the throttle a bit, I had the same problem. I changed the choke slides and the WO bog went away. Still rich overall though.

Thanks again!!!!!
Tom