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Tuning - Original Comet Carb and LM-1 AFR Meter

Pharquarx

Well Known and Active Forum Website User
Non-VOC Member
My 50 Vincent Comet (Ex EFX 585, now California Year of Manufacture 35 471) came with two carbs, the original brass pre monoblock carb (Amal 229/1L2) and a new Mk1 Concentric. I have decided to run with the original brass one because I like the look. Hooking up an Innovative LM-1 AFR Meter using their mounting device yielded initial measurements on the order of 35 to 45 lbs of air per pound of fuel, which is leaner than a popcorn fart. I was surprised that the measured AFR was that far off, the goal being around 13.8. I adjusted the needle, moving the slide from the top notch to the bottom notch. I also noted that the needle jet is 200, the throttle valve is perhaps a 2. When I remounted the carb and started the bike up again, the AFR dropped to the order of 20 to 23. Still too lean. I can make the meter read down to 14.7 and less if I partially cover the carb air inlet cone with my hand. When the Comet is shut down, the AFR reading rises to 20.9% O2 within a minute so it seems to be operating correctly. I plan to pull the muffler off tomorrow (Friday) and move the measurement point to the exhaust pipe outlet. Probably will scare the gardeners and the old lady across the street!!!

Other things to note about the carb installation is that I placed a layer of metal tape around the mating surface with the carb to positively close off any air leakage at that point. Clip ring is tightened before testing. The adaptor to head piece has the correct gaskets and are sealed with a high temperature permatex to eliminate air leaks there. The throttle and choke entries into the top of the carb have also been sealed off with permatex. In short, I do not believe that I have air leakage at the inlet.

I am really surprised by the lean readings, all of the jet, clip placement and slide numbers appear to be at the recommended points. My starting point for adjusting the air screw is one and a quarter turns off of bottom. Failing to obtain the desired readings, my next step will be to obtain a larger needle jet, I assume that one on the order of 250 can be obtained. I have all of the requisite literature to reference (Richardson, Stevens, Irving, Owner's Manual) and am going through that. The questions to the membership are, am I overlooking anything obvious, or have you had similar experience and, if so, how did you resolve it?

Thanks in advance for your input.
 
M

mogenseriksen

Guest
If you read on page 15 in your LM 1 manual you will see the reason
why you get a lean reading.
A single cyl. engine sucks air back into the exhaust pipe between
the strokes,
You will need to weld a bung for the sensor into the exhaust pipe
before the silencer.
Good luck.
Mogens
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Gas analysis

I think Mogens is right. I have a Manx. It showed perfect fuelling from 5000 to 7200 at 13.8, at which I saw max bhp (52) and torque (41 lb.ft). The exhaust sniffer was stuck a foot down the megaphone.
Note that 13.8 is OK for a race motor, but would be regarded as very lean in a road engine. Your jet size isn't far out, nor your slide. My twin runs very smoothly with 180 in both pots, #4 slides, and the needles raised one above centre. Might be better with #3 slides. (Not applicable to Comets, but the biggest single improvement in running was obtained by getting both float levels the same. No more sooty front plugs...) It still backfires for a couple of hundred yards when cold 1) which tells me I've got it about right and 2) warns mothers to hide their daughters. Only one of these is good.
Note that rich doesn't burn holes in pistons: weak may do. And use plugs hot enough to burn deposits off - NGK 5 is OK, but I use NGK 6 because i have lots of them.

Tom
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Testing motors

Roy: you remind me that I once tried a Colortune plug in my Goldie, expecting to see the clear light blue flame of perfect combustion. What I saw was closer to a fireworks display, mostly yellow. Since it would readily pull 120 mph (it was in a Seeley frame, about knee high to a small dog, and weighed 300 lb) I decided to leave things the way they were. I should have said that the fuelling results on the Manx engine were at full throttle in top having spun the dyno drum up to speed through the gears. I wasn't interested in intermediate openings, since Race Plan A is to get to wide open within the power band as quickly as possible, to which end the twist grip is as progressive as a light switch.

Tom

Before trying to re-invent the wheel, does this machine run as badly as the "test" results suggest?
 

Pharquarx

Well Known and Active Forum Website User
Non-VOC Member
Hi Guys and Girls, Frogs and Frogettes,

An update.

I extended the intake port of the Innovation mounting device with some copper tubing an additional 8 inches into the exhaust pipe. This is still with the muffler on. I also used a strip of metal tape to cover the bike exhaust pipe to muffler junction. Next I obtained some flame resistant fabric (here in California until just recently, kids pajamas were required to be made out of this type of material - settled on space ships and aliens in flannel) and draped it in a tent like fashion over the exhaust outlet and sampling device.

The exhaust AFR meter now reads at around 15.1 at best, still a little lean. This is with a 200 main jet and the needle fully raised. Well within striking distance of a 13.8 AFR with a larger main jet. I also looked at and measured the slide, it is actually a 4, so I will go to a 3. The stainless steel (VOC Spares) exhaust pipe on the bike has also blued, which suggests a significant move in the direction of a rich mix. Before, it would yellow only.

As far as performance at the current settings, the bike pulls strongly up through the gears. There is still some spitting back through the carb at idle.

I plan now to get the larger main jet, maybe a 250, a number three slide and I also want to extend the exhaust port on the Innovation mounting device to make sure that the flow through the Innovation measuring device is from the muffler, past the sensor and out the exhaust. I will use a smaller diameter copper tubing on the outlet and make sure that it extends to a point outside of the exhaust pipe.

Thanks for the input to date. Oh, by the way, I am running 87 octane now and using a KLG FE70 plug. The bike is equipped with a BT-H magneto setup. Would anybody care to opine on the plug type also?
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Jetting Comets

As in real life, rich is good. (I've been rich, and I've been poor. I prefer rich.)

I've seen it suggested that the optimum slide is a 3 1/2, obtained by cutting 1/32" off the bottom of a 4. Depends on whether it continues to spit back when fully warm. If it does, try 3 1/2. If not, leave it. Or try raising the needle one, to 4th notch from top.
250 sounds VERY rich. Should be somewhere in the range 180 to 200. Might need to be richer if your fuel contains a lot of ethanol: we still get to use petrol, with no ethanol, and therefore much leaner mixtures and better fuel consumption.(Our farmer's lobby is less powerful than yours...)
I think FE 70 is about NGK 5 or 6, both of which are good. Personally I think NGK 77 (there doesn't appear to be a 7) is too cold (hard). Champion N4, or if it gets oily, 5, is also about right.
Much depends on your compression ratio. My suggestions are for standard, about 7.3:1.

Tom
 

Pharquarx

Well Known and Active Forum Website User
Non-VOC Member
Tom,

Thanks for your reply, particularly on shaving a 32nd off of the slide. I had this carb body "fixed" by Andy Harden. The fix consisted of boring the body and then fabricating a new slide out of steel. I see on ebay that there are 229 #3 slides 20 thousands over available in brass. Looks like I need to pull out the calipers.

With regard to gasoline, in California we have CARB (California Air Resources Board) gasoline (aka. Gucci Gasoline). The gasoline specification allows 10% by volume ethanol and all of the gas station pumps are posted as such.

I will probably source the parts I need through Surrey Cycles in the UK as that is the only catalogue that I have.
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Jetting Comets

I run standard 289's, but the carbs have been sleeved, so I can't buy replacements off the shelf. If I could, I might try 3 1/2 slides, but since the backfiring stops within half a mile, and getting a pair of custom 3 1/2's made is just too much hassle, I'll let lethargy triumph.
(The classic method of setting fuelling on Manx Nortons, which in use are run mainly wide open, is to lean them out until they pop on the over-run, then richen them one. Plugs are "dark white" when mixture is correct. With the almost universal Gardner flat-slide carbs, that means clicking the needle down until it pops, then clicking it up one. No spanners involved.)
Just make sure that there's enough head space under the slide to clear the jet block, before converting 4 to 3 1/2, but I have successfully "converted" a 276 #4 to #3 with no problem. Did it with hand tools too, a Junior hacksaw and a file!
After much buggering about, I'm currently running stock settings. 180 jets, #4 slides, needle 4 grooves from top (i.e. rich). What I had been doing was trying to cure unrelated faults through carburation. I had worn guides, and most critically, the front float level was about 1/8" too high. Then it occurred to me (sounds pretty dim) that if all the settings were the same, and the front plug was STILL sooty, it had to be float level. Which Sid Biberman had told me months before was important. With the bike tilted over about 5 degrees in the direction of the pilot, fuel should JUST weep out of the pilot jet hole. (Screw out, of course.)
Bon chance...

Tom

Tom,

Thanks for your reply, particularly on shaving a 32nd off of the slide. I had this carb body "fixed" by Andy Harden. The fix consisted of boring the body and then fabricating a new slide out of steel. I see on ebay that there are 229 #3 slides 20 thousands over available in brass. Looks like I need to pull out the calipers.

With regard to gasoline, in California we have CARB (California Air Resources Board) gasoline (aka. Gucci Gasoline). The gasoline specification allows 10% by volume ethanol and all of the gas station pumps are posted as such.

I will probably source the parts I need through Surrey Cycles in the UK as that is the only catalogue that I have.
 
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