New head running issues

Hi All
In a wild moment of impulse I got one of those new Auto Technologies cylinder heads for my 750. The original head was a combat, plus a few skimmings since 1972, so it was praticicallly flush with the fins. Norvil years ago helicoiled the stud threads but one pulled out recently etc, etc. So I thought "Sod the expense" and Mick Hemmings supplied the newbie Aussie head.
Now for my question to you all. Running with the original head was fine and dandy (Yeh okay, I hear someone say, so why did I change it??) However with the new head the bike starts to falter at 70mph in top gear and will not pull to go more. If I drop a gear it hunts for a moment and kinda accelerates, get into top again and torque is lost and slows again.
Maybe fuel starvation? Carbs are good, no air leaks around manifold
Its lower compression with a new head now. Would that require any carburation changes? Mick Hemmings suggested change the needle jet to 107. Presently 106 jet onstandard twin Amals Mk 1.
Your thoughts gratefully received on this.

You will need to increase the mainjet size due to lower compression. Try 2 steps. Jim Comstock

comnoz said:

You will need to increase the mainjet size due to lower compression. Try 2 steps. Jim Comstock

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That you will have to explain, I would think the main jets need to be increased due to a better flow from the new head, but why would they need to be increased only with a lowered compression ratio?

All things being equal which in this case they are not, more power means more fuel. Just lowering the compression ratio would mean lower power and consequently less fuel and smaller jets no :?:

Jean

Jean, FWIW on my high compression motors with properly flowed heads, I use 230 main jets....a better flow will cause more depression over the jet, therby sucking more fuel, but needing a smaller jet!

Just guessing but that is similar let down to what happened to 10 CR Peel when I went from small port head for break in to 'proper' big port CHO head - until over 7200 rpm. Went form sport bike spanker G's to ordinary Commando left behind in the road games.

If not enough air velocity then can't suck up enough gas, so extra gas may help. Also lower CR is harmful to longer duration [2S] cam performance at lower rpm-air flows. Might test w/o base plate and/or a thinner gasket after re-jet tests.
Jim Schmid's hi CR piston are lurking to match up with new hi end head too.

Steve hobot Shiver - next 920cc Peel will use CHO head over sized for a 750.

If you open the throttle at 70 does the engine eventually start revving again or does it just refuse to accelerate at all? If you sloooowly add throttle at 70 will it accelerate? THe chokes (if fitted) are fully up, right?

You can also try dropping the needle 1 notch (raise the clip 1 notch).

Carburetor jet size is determined by the strength of the signal seen at the jet. Lower compression will reduce the strength of the pulse produced when the valve opens so to flow the same amount of fuel you will need a larger jet. Take it from the guy who designed the head. Increase your jet size. Jim Comstock

MexicoMike said:

If you open the throttle at 70 does the engine eventually start revving again or does it just refuse to accelerate at all? If you sloooowly add throttle at 70 will it accelerate? THe chokes (if fitted) are fully up, right?

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Once at 70 it just lurches and faulters, refuses to accelerate even when throttle opened slowly.

Comnoz: when you say increase jet size Im assuming you mean the main jet? Would increase in Needle jet size make any difference to anything?
Sorry not yet sure how to highlight each quote to answer. The Norton has drained my mind!!!

You need to set the main jet first before changing needle jets, if you change needle jet size before getting the mains right you will need to redo the needle after setting the mains. If you go bigger on the mains this will overlap onto the needle jet range making that range richer at wider openings so try that first.

Okay Guys
Thanks for your input. I`ll keep on tinkering. Typical though eh. Try to make improvments to yer bike, then once more involves more time and money elsewhere!!!
Was gonna go to Stafford for the classic show but Kempton Park has a classic autojumble and Surrey Cycles the carb people will be there also, so I can pick up some bits and try it out again.

You can go up or down in your jets but if you are cutting out at a point then the needle needs to be fiddled with. Mick has it right by suggesting a leaner needle. The increase in venturi is pulling more gas than can be mixed. This is regulated with the needle. You can check this by simply lowering the needle 1 notch and run it to see the change. This will cost you nothing.

Vested interest speaking here.

When I had my 850 Fullauto Technologies head fitted I didn't change the jetting on my single Mikuni VM34 at all. Mind you, the compression ratio was probably the same or close to it between the old and the new. I now have 8000 miles on it and it certainly has more grunt right through the rev range over a standard head. Once you sort out your carburation issues, I'm sure you'll find the same. In fact, I found mine to be undergeared for the power characteristics and I now run a 23 tooth gearbox sprocket and it has oodles of power everywhere.

By the way, shouldn't this thread be "Carburation issues with fabulous new head" ?

Now I'm confused, ok I mean more confused.
Two opposite schools of thought presented.
1. Not enough air flow to suck up fuel, so jet richer.
2. Too much air flow by Comstock's FullAuto head, so jet/needle leaner.

Basic Amal lore is, if picks up power from WOT by a slight back off, implies too lean, if no pick up by slight back off WOT, implies too rich.

Being a seasoned Commando owner I'd check plugs and timing next.
Be suspicious of fuel flow issue, tank drains to float bowl level.
What pilot air screw turn out gives best stable idle?

Kommando is correct in saying to change the main jet first. You may find that you also need to raise the needle a notch or two or maybe stepping up to a 107 needle jet and lowering the needle back down to where it was before the change.

A carburetor is designed to mix one part of fuel to every 10 to 14 parts of air that flows through it no matter what the velocity of the air. That works out great because engines run best when the fuel air ratio is in this range.

What changes the needed jet sizes is the way the air pulses come through the carb. Increasing the compression ratio makes the pulses of air coming through the carburetor stronger so you need a smaller jet to end up with the same mixture ratio coming out of the carb. Carburetor jetting is not related to how much power the engine makes or what size the engine is although changes in displacement, compression, valve timing, port size, exhaust sytem and a host of other small things can affect the strength of the pulsing that will affect the needed jet size.


Of course an engines fuel usage goes up if it makes more power but the size of the jet does not go along with power output or engine size.

1- If the only change to an engine is an increase in compression ratio you will normally need to jet smaller.

2- If the only change is an increase in displacement then you will normally need to jet smaller.

Both of these changes will increase the pulsing in the carb and more fuel will flow through the same size jet.

The changes in the porting on the new head do not change the pulses seen by the carburetor very much so if nothing else is changed when the new head is installed, then the jetting will remain the same.

The only time this does not hold true is if the engine has a long duration cam with a lot of overlap and a tuned exhaust system. The head has a step in the exhaust port floor which will reduce the amount of reverse flow from the exhaust system and therefore reduce the amount of "double carburetion or megaphonitis" so you will then need to decrease the slide cutaway to avoid a low speed flat spot.

Double carburetion happens when heavy pulsing from the exhaust system causes the air to move into the carb and then back out and then back in before it makes it into the cylinder and is trapped when the valve closes. Each time the air goes through the carb it pickes up more fuel and the mixture becomes very rich. Jim

Quit guessing. Jet according to plug color. If pump fuel burns too clean - use Av gas. Test each throttle setting with a plug chop. Adjust partial jettingwith needles. Wide open with main jets. If the carbs won't straighten out - trash em and get a better ones.

Jim, I'm getting slightly conflicting plug readings.

If I run with Champion N7YC (I think), I get a darker colour on the plug chop than with my NGK Iridiums. I am assuming that this is down to plug "heat" rating? The difference is not massive, but the Champions suggest the mixture is a bit richer that the NGK Iridiums. I can't check the plug No.s as I am not at home now.

I just wondered what you thought? Which plug do you run with. The engine performance and pick up etc appears to be OK.

Apologies for taking the topic a bit off track.

Reggie, The Champion plug you quoted is somewhere between a number 7 and a number 8 NGK plug. It will do fine for most riding as will a NGK 7. For hard highway riding I would suggest a NGK # 8 plug to avoid overheating the plug and causing pre-ignition. The iridium electrodes don't change the heat range. They are just able to stand high temps longer without the gap becoming larger from erosion. I just use a copper core plug like all standard NGKs or Champions that end in "C".

NGK plugs get colder as the number gets higher and Champion [and most others] get hotter as the number gets higher.

With todays fuels the plug should run nearly white because there is no lead to color the plug. If the plug is getting dark with unleaded fuel then it is either very rich or it is burning oil. Jetting a bike on todays fuels by reading the plug is difficult without using race gas or leaded av gas.

If you have the head off or have a boroscope you can look at the combustion chamber. It is just right if the chamber is medium gray and the intake valve is dark gray . The exhaust valve should be light gray. Black anywhere indicates rich mixture or oil consumption. Heavy oil consumption will wash the edges of the chamber down to clean metal. These colors should be observed after riding at speed.

With the fuel injected bikes I run an NGK #9 or 10 [very cold] plug because the burn is much cleaner and fouling is no longer an issue.

ludwig said:

Stop guessing ..get one of these :

http://www.lm-1.de/contents/en-us/d38.html

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They work great, but they are expensive and need a bung in the end of the exhaust pipe or front part of the muffler. Or you need an extraction tube to sample the gas in that area. When you try to sample near the end of the muffler the reading will be skewed by ambient air entering the end of the muffler.
On my dyno I use one with a 24 inch tube that sticks down the center of the muffler and has a small pump that draws the exhaust out and over the sensor. As I pull the tube out of the muffler the reading goes bad when the tube is still about 12 inches from the end of a peashooter.

Ludwig, As far as I know all the wideband sensors are heated. The controlled temperature is part of what makes them accurate over a wide range of mixtures. I have two LM1s and two lc1s that I use regularly along with a five gas analyzer. They are an excellent investment if you do much tuning. And your right ,they are cheap in comparison to burning an engine down. However, even with all the goodies, I have still managed to burn one down in the last year. Right Gary? Jim