|
|
Post by fozzledoff on Jun 6, 2012 8:18:13 GMT -5
Hi all
Ive recently put some Mikuni VM34s and pod filters on my 76 motor- 70s Frame bike. I was assured they were "jetted for australian climate and fuel", but wouldnt you know it, they didnt work. I changed the needle to the richest setting and changed the pilot jet from 25 to 30, then handed it over to a professional. He messed around with different settings and jets and then settled on what I had! Anyway- he told me he had to run it quite rich to compensate for the excess air flow. Which i understand. He said this might cause the plugs to foul after a while (but maybe not) which I also understand. I was just wondering if there are some hotter plugs I can use than my current BP7ES, that will perhaps not get fouled up? Mikes XS have those NGK iridium plugs.. anyone used those?
Cheers guys
|
|
|
|
Post by grizld1 on Jun 6, 2012 8:42:35 GMT -5
Your "professional" has some learning to do. There is absolutely nothing in the design of the VM34 carburetor that should force you to run so rich that the plugs are fouling. Running hot plugs to clear the fouling is not a good solution, and I shouldn't need to tell you why. Iridium plugs will not help.
No advice can be given from the information in your post. If you supply a complete jet list--and I mean with every piece of brass listed and float level specified--I may be able to help you.
It has often been noted that 70% of carburetor problems are electrical, 20% are mechanical, and the remaining 10% are carb related. Make sure there's not something else going on that's making your motor ask for a fat diet.
|
|
|
|
Post by fozzledoff on Jun 7, 2012 0:44:15 GMT -5
Pilot Jet- 30
Air screw out about 1 full turn.
Needle- 6F9 on richest setting.
Not sure about needle jet.
Main Jet- 230 or 240, I cant remember what he said. It came with 220 from the supplier.
Float setting- From the factory.
Compression is about 145 to 150 in both cylinders.
Valve clearances are within specs.
Cam chain tension is fine. All timing mechanisms have been replaced, points spacings set correctly, timing set and checked with timing light. The advance system is new also and works great. I have a Sparks alternator, and new battery, that I keep on a good charger.
My "Professional" (who works for a racing team that just came in second on the weekend in the national titles) didnt say they will definitely foul, he just said it is running on the rich side to compensate for the influx of air. He said to take it for a long ride and check the plugs after.
IS there NOT a huge air rush that comes in with round slides due to the direct connection of the throttle cable to the slide? The old carbs with the diaphragm and butterfly valves work differently right? the slide doesnt rush up in direct response to the pull of the throttle?
I was just wondering if I could put in one grade hotter of spark plugs or something simple.
But if you can tell me how to jet my carbs from behind your computer screen then go right ahead? And you shouldnt need to tell me why hotter spark plugs wont help the situation? So I should know everything already? isnt this a forum where people share knowledge?
I dont mean to be rude. You have given me some great information before and im grateful. And simply telling me that hotter spark plugs wont work, and a brief explanation of why would have made me happy.
cheers
|
|
|
|
Post by fozzledoff on Jun 7, 2012 0:59:30 GMT -5
Plus- I do admit having absolutely no knowledge of the electrical kind, regarding ohms, volts etc. But having heard that doing things such as putting the NGK iridiums in your bike, or changing to a boyer-bransdon digital ignition, or using high performance coils, or putting pod filters on your bike, usually means you have to richen the mix a little to compensate for either hotter spark or increased air flow. I thought it was a pretty fair (however naive) question to ask whether or not hotter spark plugs might help reduce the fouling a little.
|
|
|
|
Post by Tom Graham on Jun 7, 2012 9:18:23 GMT -5
Pull off the "Pod Filters" and see if the engine runs better. Some of these pod filters are so bad that they have been the reason guys are tearing their hair out trying to jet the carbs. I can only recommend K&N or UNI foam.
Keep in mind that the stock carbs, jetting, ignition all ran fine for many miles and many years. Most of the time there is correctable damage that is keeping the engine from running properly.
Tom
|
|
|
|
Post by grizld1 on Jun 7, 2012 17:48:33 GMT -5
Tom, these are not BS vacuum carburetors. There is no vacuum chamber port to block with internal flanges, and blockage of air jets will not occur.
Fozz, without information on float level and NJ size I can be of little or no help to you. I can tell you this: race tuning and street tuning are two entirely different ballgames. The 6F9 needle is fine, but with a #30 PJ and the needle at #5, I'm frankly surprised that your motor runs at all. With VM's on a street-going performance-modified XS650 (cammed, ported, bored), operating at about 400 ft. above sea level, I usually wind up with a P5 NJ, 22.5 PJ, 180 MJ, needle at #3, float level in the middle of the range. This isn't a recipe to follow, just an indication of how extreme your jetting is for street use. Variations like humidity or a few hundred feet up or down in elevation aren't going to make that much difference.
Re. hotter plugs--they may burn off the fouling, but if you're running hard, they won't be kind to your piston crowns. Iridium plugs are more vulnerable to fouling than the plain-Jane BP7ES.
Yes, mechanical slides lift faster and admit more air through the venturi than vacuum carbs, leading to momentary leaning out in fast transitions. That's why pumper carbs were developed. That having been said, you shouldn't notice any adverse effect on performance in comparison to the OEM vacuum carbs--in fact throttle response should be quicker. Any carb will bog the engine if you snap it open instead of applying a quick roll.
"From the factory" is not a float level specification. I've written this so often I'm sick of it, but when carbs are shipped there's no fuel in the bowls to damp the motion of the floats and the carbies get tossed around plenty, so what left the factory may not be what reached your door. Read the level at the point on the float arm where it contacts the horizontal pin on the float.
|
|
|
|
Post by fozzledoff on Jun 8, 2012 2:10:35 GMT -5
OK Thanks for that.
I did check the float level when I originally had problems with the new carbs, and they were between the range specified in the tuning manual (which I cant remember now),
What country are you in Grizld? I heard the fuel difference between Aus and USA makes a big difference to jetting. Ill have a look at the NJ and the float levels when I next get a chance. Thanks for the valuable info.
Regarding the extreme jetting- I actually have no idea what was done to the motor by the previous owner who rebuilt it. I dont know if he bored it out or polished the ports or what..
The only reason I opted for the VM34's and pod filters(K&N), was that my bike being a 76 motor and 70s frame didnt fit together very well.. and on long trips the air boxes would pop off the back of the carbs allowing too much air flow. Im not interested in breaking any speed limits or reaching them in record time... I just wanted a bike that was slightly more reliable. Arent I the fool.. once again.
F$%K this nuts..
|
|
|
|
Post by Tom Graham on Jun 8, 2012 9:00:06 GMT -5
Good point Griz. I've seen some cheaply made, very small pod filters. They are so small that these filters look to flow much less than the stock air box. Maybe they are merely screens and flow well but not filter? I don't know but I hate the ugly things and I see them too often. Easy to pull off and see if it makes a difference.
A hotter plug will help diagnose problems. Important to use premium fuel for the higher octane. To be safe don't run the engine hard, that includes long freeway blasts. There have been hotter plugs recommended by mikesxs and Pamco Pete for daily use so they should be fine for diagnosis. The BP7ES and comparable plugs have provided good service without problems so tune for this heat range plug eventually.
Don't forget that the idle jet can pick up a tiny bit of dirt to block its flow. Most of the time it shows as one cylinder not idling but I've seen bikes with the carbs running off the needle because the idle jets were all plugged. You can pull out the mixture screw and spray carb cleaner inside with the little tube. This should blast the dirt back into the float bowl if there is any. Another quick thing to do.
Put a timing light on your engine. I see 1976 and think points and mechanical advance. Time for full advance and double check that the new advancer isn't sticking. Weak or badly mistimed spark can show up as a rich condition.
Tom
|
|
|
|
Post by grizld1 on Jun 8, 2012 9:31:55 GMT -5
Yeah, Tom, those little conical pods have nothing under the screen but a very thin layer of foam--good for screening gravel, nothing more.
Fozz, I'm in the States. Right, there are fuel differences, but what you have in those carbies is close to what might be found in a full-race setup over here.
Here's what has me scratching my head. There are very strong interactions between PJ selection and needle clip position; even moving the needle a single step often forces a compensating change in the PJ. So you've fattened the idle mix big-time with the needle as well as the jet--but your air screw setting is telling you the mix is in range! That's what had me wondering about float level, although defects there usually show up only at wide throttle openings. If level was good when you measured, it's good now.
Maybe some of the guys in the Australian 650 club can help. Have you been in touch with them?
|
|
|
|
Post by fozzledoff on Jun 10, 2012 21:11:58 GMT -5
I havent been in touch with the XS club of Australia yet.. perhaps I should.
Im baffled then.. unless my bike has been hugely modified by the guy that rebuilt it a while back.
So are the K&N pod filters a bit useless? Should I look into the UNI foam filters or one of those foam socks that fits over the existing pod filters? I wont take up anymore of your time..
thanks for all the help..
|
|
|
|
Post by grizld1 on Jun 11, 2012 11:38:00 GMT -5
Fozz, if you have K&N's, they're fine. The filters you don't want are the cheap cones with foam under the screen. Uni's are OK unless you get caught in the rain. With the K&N's, Outerwears rain covers will eliminate any moisture problems. They don't stock covers for every application but will make up covers for any K&N filter for a reasonable buck. Google for a URL.
The Australian 650 club is full of tech-savvy hands who'll be ready and able to help; they're great guys. Wish I knew more about effects of fuel differences, but I can't imagine they'd be quite that extreme. Do check out your NJ size; that bit is the heart of the carburetor. Good luck, and please school me on what you find out!
|
|
