Griffon twinport settings.

[dave higgins]

I`m on the borrow again chaps, i`ve always had running problems with my twinport, & have stripped it as some of you will know, but i have not found anything to cause the problem.
So i need to reasemble it to its original spec & start from there, i need base gasket thickness, carb settings for what i believe is the original 32mm mk2 amal. any other info greatfully recieved. regards, dave.

[scott151]

Twin port

Hi Dave,

What was the problem with the twin port originally?

The twin port 380 would have had a mk1 932 Amal concentric originally.

I believe they had a 106 needle jet, 300 main, 30 pilot, I will chech my data book for the throttle slide number, I think it was no3 from memory.

I will check to confirm.

As to base gasket thickness you need to assemble the engine and measure the distance to the top of the Barrell. Kim seems very up to date with squish clearance etc.

Personally I would leave it on the lower side of compression ratio to help relieve detonation

Scott

[dave higgins]

Hi scott, sorry i meant a mk1, thanks for pointing that out, yes, thats what i need to know, port timing etc.
The problem was, starts first kick, runs fine, revs out instantly clean & crisp, idles nice, but, when you pull away, it feels like you are in 4th gear.
Reving is the name of the game, once you are moving at a reasonable rate, performance is rapid, everything on the bike is as it should be, gearing, engine condition etc.
Carb is new & have changed jetting numorous ways, the only thing i keep coming back to is, port timing, has someone tuned the motor, to be at its best, only on the top end or something.
Either way, what i am after is, the basic factory settings, so as i can see what has been or has not been changed. thanks for your input scott, regards, dave.

[scott151]

Twin port

Hi Dave,

We had a similar fault with dads Hawkstones last week as gosfield, the little sod was playing up all flipping day, dad just could not get it off the line, we found the fault in the end, it was an ignition fault in that the kill button was earthing out under load?

What ignition are you running mate?

Have you checked that you getting sufficient fuel to the carb under load?


Scott

[dave higgins]

Hi scott, i`m using a big bore fuel line, with a methanol float bowl, which has double the flow rate of a petrol unit, the tap is standard, & the ignition is new & is simons electronic pvl.
It will get away ok, but only on high revs, once away, it rides as normal, it seems to have the symptoms of running to weak, but the plug is burning a nice light brown, & if you richen it up anymore, it starts, four stroking coming out of corners.
I`ll find it, i`m sure just rebuilding it correctly will eliminate the problem, thanks anyway scott. dave.

[scott151]

Hi Dave,

Could it be getting to much fuel? As at high revs it can keep it clean.

It might be worth reverting to a standard float bowl and fuel pipe and re trying it mate. If its four stroking when you richen it up, it might be because it's running rich already.

I only say that because if it starts first kick, it must be abit rich to do that.


My 24mx4 short stroke and Hawkstone run fluffy when cold but perfect hot.

Dads Hawkstone starts first kick but is always smokey and oily, but his has had abit of breathing on

I have a spare Barrell if you would like to try it?

They are annoying to get right though mate.

Best of luck with it

Scott

[Andy Z]

Hi Dave,

I had a similar problem last year. I found that it was running too lean on the pilot jet and in the end used a 40 pilot jet and raised the needle one notch too. After that it would launch like a rocket until the gearbox packed in! Even got the holeshot at Maylandsea in one race.

The spark plug colour is near-perfect, ever-so-slightly rich.

In theory, my barrel should be the same as yours, 380 twin-port, 32mm MK1 carb (new). Settings are:

MJ: 300
PJ: 40
NJ: 106
Groove: 3
Slide: 3

Hope this helps,

Cheers,
Andrew

[dave higgins]

umh!!, i tryed the 40 pilot andy, no difference, but i don`t think my main jet is as large as that, i`ll have to check.
Scott, the larger float needle appature & fuel line only aid filling, the carb will still only draw what it needs, but i know where you are coming from, & again many thanks for ideas.
I will try all that has been suggested as soon as its built back up, & hopefully, make some progress, but half the trouble is finding somewhere to test it out, you can only go up & down my street some many times, before you feel you have a target on your back.... dave.

[Kim275]

Hi Dave - if you are worried about port settings being non-standard - the best way to check the barrel is to do a port map trace and then do same on a barrel you know is not modified...unless someone can provide the port measurements for a twin port . The measurements you need to check - relative to TDC - are the exhaust and transfer port heights - measured form top of port to TDC and the inlet port height - which is from the BOTTOM of the port not the top.
If Andy could do a trace of his barrel and mail it to you to compare that would
help.

The things that at least I know of that could cause the performance problem you have - form a porting perspective are
1. Exhaust port timing raised....(port higher - so measurement form TDC to top of port smaller than standard) will increase your revs and decrease bottom end power (with no power gain on top end unless your exhaust modified to match
2. Inlet port timing raised - either by cutaway on back of carb or grinding on bottom of inlet port......This will increase top end power but kills the bottom end pick up .....Terry Sewell did the cutaway trick on his griffon grass tracker - and it filed...but no good for motocross. This symptom is exactly as you describe...like pulling away in 4th gear......check your carb has no cutaway at the back
3. Squish velocity too low (squish band ratio lower than standard)......this has the effect of increasing top end and reducing bottom end torque. so if somebody has played around with the head this could be an issue.

Re Squish band clearance - this clearly will be affected by gasket thickness......but your net squish band clearance should be about 1.5mm - to be on safe side to avoid detonation on a 380. The 380 Griffon - has the standard tufnol thick gasket plus I think two 0.4mm paper gaskets one either side (Chris could confirm this) .

If you can get the measurements I am happy to do the mathemagics and calculate your timings

Cheers Kim

[dave higgins]

Hi kim, thanks for the info, i`m gonna build it back up this w/end, & i`ll be able to get some measurements/idea of whether its standard or not. many thanks, dave.

[riflegreen]

Kim ,

That is all I did on the works bikes , tufnell and gasket either side . Didn't alter the ports , the standard timings were more than adequate .

It sounded to me like the bike was very overgeared but you never know what a previous owner has done to the barrel .

Chris

[greeves246]

380 twin port

dave jsut try a bigger pilot jet or smaller ? to start with dont make to bigger chages a your not to far out

[Kim275]

Kim ,

That is all I did on the works bikes , tufnell and gasket either side . Didn't alter the ports , the standard timings were more than adequate .

It sounded to me like the bike was very overgeared but you never know what a previous owner has done to the barrel .

Chris

Chris - makes sense - as far as my experience goes - the Porting is the thing that is best left alone - as for the port layout design - i.e with no multiple transfer/exhaust boost ports - the Griffons are pretty spot on. The QUB power and spread is one of the best for its age.

So Dave - based on Chris';s validation I think best best plan is to do the blueprinting first confirm the ports are standard - then get the gaskets set to standard....and then finally check the squish clearance - and as long as it is not less than 1.3 to 1.5mm - to avoid detonation - you should be spot on. Any more not an issue as will just reduce your CR.....

Then measure the squish band ratio - and as long as this is in the range 40 to 55% you should be good (any less your head is set more to road racing narrow squish bands and characteristics)


Happy fettling