78 replies to this topic

[spuker1]

LIke I said before the adj fuel pressure are crap. Ive tryed useing one. Have you re jetted the carbs jet?

No I haven't but I don't think that jetting is an issue because there's no problem with supplying correct amount of fuel when reving up slowly...

Carbs with a vacum diaphragm slides (CV carbs) don't tend to like those air filters and usually cause issues.

Well... I will try running them with filters off to see if it changes anything, why not. But I don't think that under-fuelling on full throttle is caused by a piece of cotton that if it affects airflow at all it actually reduces it!

[spuker1]

Anyone any suggestions? It has to be one of those three IMO:

-Fuel pump not strong enough

-Jets too small

-Ignition timing off for some reason

Anyone could give me some feedback on that? I would love to get it running right-ish before next Sunday........

Edited by spuker1, 21 July 2013 - 03:32 PM.

[rollz87civic]

I would recommed changing the jetting or start drilling the jets  out a one size bigger at a time tell you can get  The stock pump is more than enough for grative feed carbs. Even with the vac adv not hooked up it should rev up with no issue.

[jjamiemmark]

No I haven't but I don't think that jetting is an issue because there's no problem with supplying correct amount of fuel when reving up slowly...
Well... I will try running them with filters off to see if it changes anything, why not. But I don't think that under-fuelling on full throttle is caused by a piece of cotton that if it affects airflow at all it actually reduces it!

It's not the filter, CV carbs use vacume to pull open the slide when you open the throttle. Have you looked in to the carbs as your giving it throttle.

If you take the oem air filter box of most motorcycle's using that style carb they would go no where without extensive jetting and a different shorter needle.

The little peanut shaped hole at the top of the carb inlet it where it gets its vacume from. If the slide diaphragm is ripped or stretched from ethenol it won't run very good either.

[spuker1]

Oh, I see what you're getting at with those filters. Unfortunately I can't use the stock airbox because I don't have it and also there's no way it would fit in there. There's no enough space for anything else in there.

Diaphragms look like brand new along with all the rubber seals. There's no sign of them being stretched and soggy and I've checked them for holes and they are sound... Slides are going up and down with some resistance and a nice thump/clunk sound to them so I think that wouldn't be an issue.

So, do you think that my best bet is getting the jets drilled/replaced for bigger ones?

Edited by spuker1, 22 July 2013 - 03:30 AM.

[anjin]

I thought there were two jets - one baseline and one for acceleration? 

[spuker1]

Yea, each carb has got two jets, I think the smaller one is for idle etc, and the bigger one is the main one that takes over when you boot the car...

[jjamiemmark]

Do you what bike these carbs came from?

what brand carbs are they?

 

DONT DRILL, jets just get bigger ones. Have you actually confirmed they slides are opening with your finger with the engine running. After watching your vids one to many times i am still thinking your not getting throttle. The small pilot jet controls idle and just off idle, the Big main jet controls full throttle and the needle attached to the bottom of the slide does most of the stuff in between. If that needle is not coming up fast enough you will go lean real quick.

[spuker1]

They came off Honda CBX550 F2 and they are Keihin VE series carbs.

I will check it out today after work mate, if that's the issue what could cause it?

[rallyxcrx]

I got to the point with my bike carb set up where it just wouldn't run right, my motorcycle expert said I needed gravity feed, I replied " what am I going to do, mount my tank on the roof?" This is when I gave up. It can be done though, I know the kart guys have solved the problem, Dave

[spuker1]

I hope I will be able to make it run right... I suppose I should get carbs off a motorbike that was running a fuel pump to begin with... On the other hand I can't see how would replacing gravity feed for a fuel pump would make it go lean on wider open throttle... However you might have a point because there are two extra ports on my carbs that look pretty much like the fuel inlet. I've asked a guy who sold me those carbs (he works on bikes all the time and he seemed to know his stuff) and he said that they are tubes for air to pressurise (? - not sure if he used that word but that's how i understood it) the fuel tank to help it feed the carbs as it was just gravity fed. He said that I won't even have to blank the off if I'm using a fuel pump...

But if it's really the case of needles not going up when throttle is open, what would cause them to stay shut?

Edited by spuker1, 22 July 2013 - 10:29 AM.

[jjamiemmark]

there just vacuum ports used for the carb sync procedure. 

 

But if it's really the case of needles not going up when throttle is open, what would cause them to stay shut?

 

Not having the stock air box will do this.

 

These carbs rely on the vacuum caused by the engine sucking against the air filter to lift the slide up out of the carb. That is why the little port you see at the top or side of the carb mouth is connected to the chamber above the diaphragm-go and try it; if you suck on that little port, the diaphragm pulls the slide up! ( use a straw!!)

If the draft from the engine cannot lift the slide (either 'cos the diaphragm is split, or there is no air filter to suck against) the slide stays down, and you have no power! Usually if you rev it hard enough, the slide will eventually pop open, with a burst of air and fuel, giving the impression that there is lots of top end power, when in fact there is merely no mid-range!

If you are determined to fit cone filters to CV's you have to fool them into thinking there is still a filter box. Ledar corrector kits fitted a restrictor to the Diaphragm port(sometimes with an extension tube) dynojet do something similar. jimmy riggers can cover about 70-80% of the cone filter's surface with  tape. This looks ridiculous, but works very well. Leave the "hole" in the tape covering at the bottom, or they fill up with water!

Later carbs(Post XS1100!) have the diaphragm ports on the other side of the carb ie, on the engine side of the slide, not the filter side! this means that the carb is little affected by the filter; more air coming through opens the slide more, and (by lifting the needle!)provides more fuel. This makes these carbs very adaptable; almost self-adjusting!

This is why lots of car tuners are fitting bike carbs, with great results!

[spuker1]

Would there be any other solutions then getting newer set of carbs or taping over the filters?

[jjamiemmark]

Probably but i would start with the tape and see if make a difference first..

[spuker1]

Ok,

So I've taped the filters up with good old duct tape and left about 0,5cm gap around the wider end of each cone filter clear. It idles 14,5:1-15,5:1 and it revs up all the way through rev range on between 12:1 - 14:1 (I think). It revs super quick and super smooth now and idle is beautiful. All it was is too much airflow, restricting it helped a lot.

Just for science's sake it couldn't be because of vacuum generating before the throttle body, there's no vacuum there anyway and the needles were going up and down as normal. Just too much air that's all.

I will get the video up at some point tomorrow probably!!!

All I have got left to do is to reinforce throttle/choke cable bracket and get it all looking neater.

Thanks a lot and stay tuned!!!

Edited by spuker1, 22 July 2013 - 03:54 PM.