JMB Diaphragm thickness

[Guest]

Although the carbs are still off my bike, I have installed a set of JMB diaphragms. This comment/question is primarily for those who have already done this themselves.

The diaphragms are noticeably thicker and stiffer than the OEM ones. When I push the slides up fully and let them drop, they do so very slowly. The originals would drop quickly with a gentle "thunk". It is as if the stock springs are not strong enough to push the slides back down firmly. This happens to a varying degree on all 4 slides. On #4 the slide did not fully seat on its own until I "helped" it down. The carb bores and slides are clean & the slides moved smoothly before I attached the new diaphragms.

Perhaps this will sort out when the engine is running due to the way air flow below & above the slides/diaphragms works. But in looking at this action while they are on the bench, it seems if they hang open like this, the engine will be screaming if I blip the throttle.

If you have installed JMB diaphragms and noticed this when you first put them in, I'd really like to hear from you. I hadn't read any reviews that commented on this being a problem but having dealt with stuck open slides on a running engine in the past I hope to avoid this coming up.

Thanks as always & ride safely,

DH

[Steve]

..., it seems if they hang open like this, the engine will be screaming if I blip the throttle.

I have not even seen these diaphragms, let alone installed any, but I can assure you that it won't matter if you prop your slides fully open, ... your engine will NOT scream if you blip the throttle.

The slides have NOTHING to do with controlling engine speed, that job is handled by the butterfly valve that is attached to the throttle cable.

The purpose of the slides is to provide a variable venturi to keep air speed up across the jets, so you don't need an accelerator pump.

.

[Guest]

I have not even seen these diaphragms, let alone installed any, but I can assure you that it won't matter if you prop your slides fully open, ... your engine will NOT scream if you blip the throttle.

The slides have NOTHING to do with controlling engine speed, that job is handled by the butterfly valve that is attached to the throttle cable.

The purpose of the slides is to provide a variable venturi to keep air speed up across the jets, so you don't need an accelerator pump.

.

Thanks Steve. You know the more I learn about anything or the more I get better at doing something, the more I realize how much further I have to go in my education. So while I do well with what I call "PRACTICAL PHYSICS", that is, if I do A, B happens, I know squat about chemistry, physics, etc. I still don't buy into Bernouli's Principal keeping a 747 up in the air because the air is moving slightly faster over the top of the wing. I am pretty sure it does tho. Try not to fly too much tho just in case the Universal Accountant realizes it doesn't make sense and I'm at 36.000 feet that day the "adjustment to policy" is made.

So that is why the thicker & stiffer diaphragms work as well as the thinner OEM ones. OK, back to class for me,

Thanks!!!!!

DH

[Big T]

Well, go from Bernoulli to Venturi and you'll figure it out, carb wise

[Guest]

Well, go from Bernoulli to Venturi and you'll figure it out, carb wise

Bernoulli?,,,,,Venturi??????? What gives here? Weren't there any Irish, Germans, Poles, Scots or Residents of Pago Pago involved in air flow? Just Italians??? Have to ask my wife how come she doesn't have any relatives to have air flow principals named after them. Well for whatever as long as it works on my Japanese motorcycle I guess it's OK. Global transfer of technology I guess.

DH

[Big T]

Well, as far as physics goes, the Germans mostly came later, with light and nuclear and such

But, the pressure drop i n the venturi is what moves the slide in a CV carb. Note the velocity stack that is the carb to airbox boot, smooths the airflow and more pressure drop

[Gorminrider]

The diaphragms are noticeably thicker and stiffer than the OEM ones. When I push the slides up fully and let them drop, they do so very slowly. The originals would drop quickly with a gentle "thunk". It is as if the stock springs are not strong enough to push the slides back down firmly......

Whatever, your description would worry me too...If the new diaphragms are resisting the original springs...it would seem they will react to variations of vacuum differently than the originals.

Also, glancing at the web-page, it seems centred on Yamaha Xs650s ...TWINS with each cylinder being 325cc..who knows what vacuum they produce?

[Steve]

Also, glancing at the web-page, it seems centred on Yamaha Xs650s ...TWINS with each cylinder being 325cc..who knows what vacuum they produce?

The vacuum that the slides respond to is what is created by the air rushing through the venturi under the end of the slide, NOT by the piston pulling against the throttle butterfly.

The vacuum is created when air has to rush through the smaller area that is created by a lowered slide. As the throttle is opened, more air has to go through there, which lowers the pressure in the venturi. That vacuum is transmitted through the hole in the bottom of the slide to the area above the diaphragm, and that is what lifts the slide. If the slide lifts too much, the vacuum will be less, which means less pull to raise the slide, so it drops. It's a self-regulating system, but it's possible that a diaphragm that is too stiff can affect response time or actual opening.

.

[Guest]

Whatever, your description would worry me too...If the new diaphragms are resisting the original springs...it would seem they will react to variations of vacuum differently than the originals.

Also, glancing at the web-page, it seems centred on Yamaha Xs650s ...TWINS with each cylinder being 325cc..who knows what vacuum they produce?

Thanks Big T, Gorminrider & Steve. Lots of good info & food for thought. In the end now, the proof of the pudding,....etc. So far any feedback of I have read on these has been 100% positive. The seller has stated that I am the first one to contact him with this concern and has never gotten feedback that they did not work properly. Guarantee?? no, hope? Yup. I have much more work left before the engine will be fired up but will get back to GSR as the truth be knowm.

Thanks again to all & sundry for your communications & suggestions.

DH