Shim Business in a Head Swap

What is the best way to handle the shims when I am doing a head swap ?

I am Swapping the head on my maxim XJ650J. The Head I am swapping with comes from a Maxim XJ750J. It has the same head but since the cams are different, I will be using my current cams (from the xj650 head) with the new head. I have both sets of cams but I was told from members in the forum that the xj750 cams will affect performance so it is best to stick to my original cams.

I will make sure to check the clearances with the plastic gauge to ensure the tolerance are acceptable.

Both heads are complete with shims, cap and cams.

I have 2 main questions:

1. Do I just swap only the cams and after everything is bolted properly , test my clearances and adjust the shims accordingly or is it better to swap my current shims and cams ?, Will one way be more efficient than other ?

2. I also want to make sure the XJ650J and XJ750J both have the same type of valves.

I will also be replacing the valve seals on the swapped head.



Thanks

 

Camshaft specs taken from XJ4Ever catalog:

XJ650 all models except Turbo:

Intake Lift: 8.50mm
Exhaust Lift: 7.80mm

Duration:
Intake opens 34-degrees BTDC
Intake closes 58-degrees ABDC

Exhaust opens 66-degrees BBDC
Exhaust closes 26-degrees ATDC

Overlap: 60-degrees


1981-83 XJ750 models:

Intake Lift: 8.80mm
Exhaust Lift: 7.80mm

Duration: not given

Overlap: not given

The only difference between the cams is the .30mm increase in lift on the intake of the 750 camshaft. Will it affect the performance of the engine? I would be seriously disappointed if it didn't!

Increasing the lift on the cams is an old hot rod trick to increase the performance of a given engine. 0.30 mm = 0.012 in. This is a mild lift, and should cause no problems.

The 750 cams will give you a stronger running engine throughout the full rpm range, but more noticeable in the higher rpms. These engines are high winding, and will benefit from the increased breathing ability in the higher rpms.

Will you have to rejet?
By increasing the ability of the engine to fill the cylinders. You will in effect be creating a lean condition by increasing the amout of air taken into the cylinder. To take full advantage of this you will need to do the full schedule of carb checks, and adjustments (Sync, Colortune, Plug chops).

When you do the carbs you will be doing all this anyways, but not anywhere near as bad as using pods! Take even further adavantage by upgrading to a better flowing air filter (K&N, Uni).

You have a good performance mod right in front of you.

You are going to be changing clearances when you change the cams. You will need to start at a base point. Then measure, and adjust the clearances to spec.

Bolt the cams in. Then check, and adjust your clearances. No reason to use the shims from the old head unless your adjustments require one of the shims. When you change out the valve seals, and lap the valves you will most likely be changing all 8 shims anyways.

They are the same.
4H7-12111-10-00 Yamaha VALVE, INTAKE
4H7-12121-10-00 Yamaha VALVE, EXHAUST

Used on all 80-83 650's, and 750's Maxim, Seca, and Turbo

Ghost

 

On the other hand, and to answer your question:

When you lap the valves and change the seals you're going to throw the clearances off. Theoretically, the shims from the replacement head would "fit" its valves. I'd use the shims in the replacement head as a starting point (keep track of which came from where.) But JUST as a starting point, you have no way of knowing whether they were in spec either.

 

Ghost... I really appreciate the detailed explanation..... From reading this the performance gain of keeping the 750 cam may not be that significant ?..did I get that right....

also I had a carb teardown not too long ago....so I was thing not to re-jet them right now.... so I would probably swap the 650 cam into the new head keep the current shims and buckets and use it as a reference point as bigfitz is suggesting.

Next time I tear-down my carb, I will get rejet to 750 spes and possibly swap the cams... Hope I am not missing anything...

Was also really curious if anyone here has swapped the 750 cam into the 650 engine and re jetted the carb.... wanted to know what kind of performance difference they noticed.....

 

Since you're filling a smaller hole the extra lift might not get you much on the 650.

 

It'll just make it more fun to get the carbs right; because neither the 750 jetting nor the 650 jetting will be spot-on any longer. I'd also be researching things like jet needles to see what other differences there are. The 550 Seca and 550 Maxim (different cams) have different jet needles and different pilot air jets, IIRC. (Too cold to go out in the garage and look it up.)

Personally, I'd keep the 650 cams with the 650 motor and get about the business of making the bike run the way it should in stock configuration.

 

Your putting the Cart before the Horse ... having the Engine and Tuning fit the Cams configuration.

Do at Fitz recommends.
Begin by building and fine-tuning a stock engine to perform at the zenith of its design and stock limitations.

Then, ... make your stocker a go-faster.

Gather the complete tech data and valve timing specs for stock camshafts and send the data plus a set of cams to the legendary "Camfather".
Iskendarian.
Isky. For short.

Have the Cams customized to increase the performance of the Plant just a few hundred-thousandths short of Critical Mass with double-warp speed capability allowing you to accelerate so fast you'll be going back in time.

Contact Name:
Richard Iskenderian

Mailing Address:
16020 South Broadway
Gardena, CA 90247
Phone: 323.770.0930
Fax: 310.515.5730
Email: richardisky@aol.com