Standard Carburetor intake manifold restriction

Bozo · February 24, 2011, 05:50:26 AM

I just took my carbies off and to my amazement the Rubber intake manifolds tapered down from approximately 36mm to 24mm at the engine. I have a 1990 model FJ1200.

My question is has anyone increased the orifice to at least 34mm (which is the size of the head intake). Does the modification drop torque etc/ increase topend/ etc. Please let me know if you made this mod and what effect it has on the power output. I do have megacycle street (mild) cams.

Arnie · February 24, 2011, 07:28:25 AM

You may have a bike that was originally sold in Japan. The FJ's there had these restricted intake runners to hobble them a bit. They may have also delivered bikes like this to some other markets. If you are planning to ream them out to full diameter, check carefully as I seem to remember someone saying that the bores are not concentric and that it would be easy to ream right out the side.

Arnie

Harvy · February 24, 2011, 03:43:04 PM

Quote from: Arnie on February 24, 2011, 07:28:25 AM
You may have a bike that was originally sold in Japan. The FJ's there had these restricted intake runners to hobble them a bit. They may have also delivered bikes like this to some other markets. If you are planning to ream them out to full diameter, check carefully as I seem to remember someone saying that the bores are not concentric and that it would be easy to ream right out the side.

Arnie

Yes. Mine had the Japanese intake manifolds, they are 28mm. I replaced them with OEM 32mm like the rest of the world got.
Ned did some research into opening up the Japanese item, but I'm not sure what his outcome was.

Harvy

weymouth399 · February 24, 2011, 03:50:17 PM

Bozo

I have some of them, if you can't find any around. Where ever you are.

Bob W

Bozo · February 24, 2011, 08:23:12 PM

Thankyou all for your replies, I'll modify my profile in the future (if I can work out how to do it) to add country details etc. I reside in Western Australia so my natural impression was that the Australian models were not restricted. The bike goes extremely well but I always found the mid to topend "flat". I have routed the intake manifolds to match the intake ports.

originally I quoted 24mm restriction but that was a quick approximation,so 28mm is about right.

Now my question is, do I need to modify the jetting, e.i pilot, and main. I also assume the flat topend should have more punch.

Harvy · February 24, 2011, 08:53:45 PM

I was running DJ stage 3 when I swapped mine.....did not have to alter any carby settings.
Please detail how you went about routing them.

Cheers
Harvy

Harvy · February 24, 2011, 08:57:32 PM

Correct me if I'm wrong, but do I see a reflector hanging down below the rego plate in the picture...... as far as I know this would suggest a grey import Japanese domestic market bike..... also, if your speedo only goes to 180kph.

Harvy

Bozo · February 24, 2011, 10:47:21 PM

Harvey, that is a reflector under the rego number plate, the speedo I had originally had the normal (240kph I think) kilometer speedo. There were two previous owners but I believe the FJ is original in every way (till I got it that is)

Also to route out the restriction I had one of those drill bits with dimples on the out side (10mm dia x 25 long) it just chewed out the rubber without gouging. The surface is a bit rough, but once filed it isn't too bad.

I also don't believe in mirror finishes on the intake, just ask why they rough sand paper the americas cup boats under the water line. Each to his own I guess.

racerman_27410 · February 24, 2011, 11:32:08 PM

IIRC the restriction goes all the way thru the carb joint... and there is an indention molded into the rubber to prevent the thing from being enlarged (vacume leak)

just trying to remember its been a long time since de restriction has been discussed.

Kookaloo! (i hope you get yours back)

Harvy · February 25, 2011, 01:27:25 AM

Quote from: Bozo on February 24, 2011, 10:47:21 PM
Harvey, that is a reflector under the rego number plate, the speedo I had originally had the normal (240kph I think) kilometer speedo. There were two previous owners but I believe the FJ is original in every way (till I got it that is)

Also to route out the restriction I had one of those drill bits with dimples on the out side (10mm dia x 25 long) it just chewed out the rubber without gouging. The surface is a bit rough, but once filed it isn't too bad.

I also don't believe in mirror finishes on the intake, just ask why they rough sand paper the americas cup boats under the water line. Each to his own I guess.

Hmmmm interesting...... I thought the reflector hanging of the bottom of the back mudguard was peculiar to Jap only models........So if its got/had a 240KPH speedo and it was OEM then its not a grey import.

As Frank said, there was a reason that the restricted intake manifolds could not be enlarged, but I don't recall the reason either (Ned may chime in here - he was investigating the possibility). But it seams you have been successful with it. Did you hog it out all the way through, or just on the head port end?

Yes I agree with the last comment regarding a non mirror like finish - the same applies to the head ports. Its the same reason they put dimples on golf balls I suppose.

Cheers mate
Harvy

Bozo · February 25, 2011, 03:25:16 AM

Harvey, I only removed rubber from the centre to the mounting flange. I did not make it straight through, it was contoured with similar wall width to the front section. it has a similar profile to the intake port. The rubber was ground to suit the intake head port without leaving any ridges.

I have just cleaned the carbies (air pressure) and refitted them, the down low response is crispy with no hesitation and the revs drop immediately when blipping to 5000rpm. So far so good, I have to fix my car before I go for a ride tomorrow. I'll keep you posted. I hope the pickup to the topend has improved considerably especially with the mild Megacycle cams.

Bozo · February 25, 2011, 03:30:42 AM

Quote from: racerman_27410 on February 24, 2011, 11:32:08 PM
IIRC the restriction goes all the way thru the carb joint... and there is an indention molded into the rubber to prevent the thing from being enlarged (vacume leak)

just trying to remember its been a long time since de restriction has been discussed.

Kookaloo! (i hope you get yours back)

Racerman, sorry missed your comment, the tool I used just ground the brass vacuum tube, it did need too much removed as I said I contoured the manifold - not making a direct hole as I believe that would cause weak points on the rubber. To be honest my manifolds are cracking and will need replacing.
I get the feeling Harvey was right about the grey import, the speedo might've been changed by the first owner.
Thankyou for your help

Bozo · February 25, 2011, 05:08:34 PM

Took my test ride,,,,,,,,,,,,,WOOOOOOOOW, talk about a change, the bike starts first kick, bottom end 1000 to 3000 rpm is the same as before, to 4500 it has slightly more power but after that make sure the wheel points straight, with an empty tank the bike lifts slightly just using power (no forced lift). The throttle response is smooth with no hesitation till 10,000 (don't forget mild cams). If I took a guess I'd say at least 10 to 15hp increase at the wheel.

Now I'm worried the clutch will have a hard time as I believe the yami clutch barely copes with normal power.

Mark Olson · February 26, 2011, 01:07:32 AM

time to double up your clutch spring.

Bozo · February 26, 2011, 07:36:32 AM

Mark, yes you are right, its time I read the section about clutch improvements. I assume you are talking about changing the clutch spring plate setup for an APE spring and hub type?.

I'll be going on a sunday morning run tomorrow, I'll let you guys know if the fuel economy improved (with some luck).

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Standard Carburetor intake manifold restriction