Delay on Acceleration

[Peder_y2k]

I've got 2 Ventures, an '83 with YICS and an '88 without YICS. Twisting the throttle wide open, the '83 accelerates briskly and without delay from a starting slow rolling speed, but the '88 seems to pause a moment before full on response, with no popping back through the intake or other indications. Almost like the machine is thinking about what the twisting of the throttle is all about. The carbs are clean and well balanced, pilot screws adjusted well, fuel level correct, engine starts quickly cold or warm, and has a nice steady idle.

Is this delay just the nature of the beast, or should I expect instant response like my '83?

-Pete, in Tacoma WA USA

'83 46K miles Avon tires

'88 52K miles Bridgestone Spitfire tires

[lsutley]

Pete,

 

My 86 had the same affliction. Thanks to the gang on here I began looking for a vacuum leak between the carbs and the engine. I found 2 or maybe 3, fixed them and all was better. Had to resync the carbs afterwards, made a big difference.

 

Good luck!!

[pmelah]

welcome abord Pete

 

these crazy loones on here are definitly knowledgeable on thes bikes and can walk you through just about any prblem blindfolded on these bike i am returning a 84 venture to its former glory with a lot of TLC and know how from these guys as well as my 99 RSTD its the best 12 bucks ive ever spent and the members ive met sofar are the best except for yammerdan im still waiting for him to tell me what size tow rope i need to use to pull my 2nd gen with my 1st gen:whistling: :bowdown::crackup:

[frankd]

Check your carb. diaphragms for holes. Since I bought my 89, it's had bog (delay) when the bike was cold and you pulled out on the highway and opened the throttle 1/3-1/2 way open. The other day I pulled mine and had holes in 3 of them. I did the temporary repair with Plati-dip and this morning it was 53 degrees and when I pulled out I had immediate response. The warm response is much better also, not to mention it's faster, smoother and quieter than before. My new diaphragms arrived in today's mail, so I'll install them Monday.

 

Frank D.

[Peder_y2k]

That's interesting, checking for holes in the diaphragms. The manual on page 5-7 clearly states to inspect the diaphragm and replace only if it has tears, making no mention of holes or pinholes or to take any action if holes are found. The diaphragm piston has a designed in hole already, so I suspect a few pinholes in the diaphragm make no difference.

However, I will Plasti-dip any pinholes I find in the diaphragms since it is very easy to do.

-Pete, in Tacoma WA USA

'83 and '88 Venture

[frankd]

I had 3 diaphragms that had about a 3/32" hole. Patching them made a night and day difference.

 

Frank D.

[Peder_y2k]

Frank, you inidicated receipt of new carb diaphragms. Where were they purchased, and what did they cost? I might want to buy some also.

-Pete, in Tacoma WA USA

'83, '89 Venture

[frankd]

jbmindustries.net and they cost $84

 

They were shipped in the mail, and here in 2 days, but they come from Ohio and I live in Illinois. I sent them an Email, and they sent me a Paypal invoice. Sirius (another place that a lot of Venturers buy diaphragms from) is out of stock until the end of the month. I haven't installed them yet, but they look good.

 

Frank

[Peder_y2k]

Found no holes or damage to any of the diaphragms. Must be another issue that results in the acceleration delay from low speed in first gear.

 

BTW, here's a picture of the diaphragm piston with the hole. Tiny holes in the diaphrgm should make no difference because there is already this hole from one side to the other in the piston. I suspect the purpose of the hole is to minimize pulsing of the main jet needle.

http://jbmindustries.com/sitebuilder/images/HOLE2-465x343.jpg

Taken from http://jbmindustries.com/Yamaha650.html

 

I guess I'll try the starting fluid spray method to check for vacuum leaks at the rubber carb mountings, even though they all are a tight fit and not dry or cracked, but one never knows what will be found.

-Pete, in Tacoma WA USA

'83 and '88 Venture

[Peder_y2k]

OK, I used the fluid spray method for vacuum leak testing, and found no leaks. No holes in diaphragms, cables adjusted properly.

I'm thinking the acceleration delay is just the nature of the beast, so i'll just live with it.

-Pete, in Tacoma WA USA

'83,88 Ventures

[gscbertrand]

Have you checked your air filter?

[frankd]

That hole in the slide base doesn't mean that you can have holes in the diaphragms and not have any effect. That hole in the slide is a precise size. The size of the tract that connects to the outside of the diaphragms is also a precise size. Now when you have holes in the diaphragms, you upset the relationship of the slide hole size and the tract size, and not quite as much vacuum is applied to the outside of the diaphragm, and therefore your slide won't move as much.

 

Frank D.

[Peder_y2k]

Thanks for sharing the in-depth understanding of the relationship of the vacuum holes/tracts and diaphragm. Now I know more than I did before.

The air filter is clean. Steady idle at 1000RPM. Carbs sync'd, good vacuum. Acceleration delay remains at about 1/2 second from slow rolling speed in 1'st gear.

What I did find with the diaphragms out for close examination, was the slide return spring for one carb was about 1/4" shorter than the other 3. I just stretched it to match the others. The thought has occurred that maybe the other three are too long and the short one is correct. If the spring is not the correct compression tension, the slide/s could experience a delayed response.

Anyone know the free length of the slide return spring?

-Pete, in Tacoma WA USA

'83,88 Ventures

[V7Goose]

As you probably understand, Constant Velocity carbs operate on totally different principles than direct linked carbs where the the slides are operated by cables or rods. In a CV carb, the slide is moved by the difference in pressure on each side of the diaphragm. Because of this, the condition of all air passages, o-rings, and vent hoses is critical.

 

I do not know a lot about the specifics of those antique Ventures, so I will simply speak in generalities. All of the ideas offered here are valid potential causes of poor performance in CV carbs. In addition, you should verify that your jets are the correct size, the slide needles are not worn or installed wrong, and most importantly, the carb vent hoses are installed/routed correctly and NOT kinked. Remove each vent hose and blow through it to verify no insects or foreign matter has clogged the line. On a CV carb, the position of the vent opening in a protected place is critical. In addition, there should be an o-ring on an air passage under the slide cap that allows atmospheric air pressure to reach the bottom of the diaphragm - those o-rings must be in good condition and properly installed.

 

Goose

[mbrood]

Springs don't get longer... they fatigue and get shorter... stretching one does not return it's original force, only it's fatigued length.

 

Remember, the piston slides in a very tight sleeve and THAT hole in the end is a calibrated relief. Holes or tears in the diaphragm alter the effectiveness and rate of piston movement.