250 in a 1969 trail grooming machine

[groomerguy]

I know zip about Ford sixes so bear with me. Our snowmobile club has a 1969 Bombardier trail groomer with a 250 in it. At least that's what's written on the valve cover. My info so far was it's a 240 Industrial engine. Apparently that is incorrect as it has an integral head and intake manifold. I did just replace the fuel pump in January and used one for a 240/300. The pump must be the same for the 250? I'm curious what the stock rocker arm ratio is and if there are different ratio rockers available that will increase my low end power without changing cams. I had mistakenly posted on the big block forum thinking it was a 240. The discussion over there is the Chevy 250 rockers (1.75:1) work on the 300 real well. Would they also work on the 250? The machine spends nearly all of it's time at 1800 to 2500 rpm and is constantly under very heavy load. It almost never exceeds 2800 rpm. It has an early model C4 behind it. I intend to do a valve job on it this summer and add headers since the exhaust manifold is warped pretty bad. I'm looking for a little more low end. I put a carb on it last summer from a 1986 F-150 300. It runs better now and gets better economy than the orginal carb. Any help would be appreciated. Thanks.

 

[CZLN6]

Howdy Back Groomer Guy:

Sorry to be so late in responding, but I've been wintering down in Sunny SoCal. I'm home now and it's dam cold in Eastern Idahol

The big six trick with chevy rockers will not work on a 250. The 250 has shaft mounted rockers like the 200. There are aftermarket high lift rockers available. Stock ratio is 1.5:1, High lift ratio is 1.6:1. You will also need to find specific pushrods for this conversion, for the 250 and for the adjustible feature of the hilift rockers. I believe Mike, our host at Classic Inlines, has both. If the engine is in otherwise good shape this is a good way to go. Be sure to thoroughly clean the rocker shaft and all oil passages while your doing the valve job.

If you're going to do the head this summer you might want to save your money and have the intake valves back cut. I think there is a picture of what this entails some where on this site. It does three things; lightens the valve train, improves flow in the mid range and give the effect of a higher lift cam.

If you're after torque consider raising the CR slightly. If the engine is still stock, you will need to mill the head .025" anyway to compensate for the thicker head gasket you will be using. An additional .025" or .030" will increase the CR to the 8.7:1 range.

You might try increasing the initial advance a few degrees too. If you haven't already, consider upgrading the '69 distributor with a Petronix Ignitor conversion. It will eliminate the points and give a stronger, more exact spark.

If the exhaust manifold is not cracked or broken it can be milled to be trued. You might check the exhaust port surfaces on the head to make sure it is flat too.

If i'm not mistaken the Carter YF carb off of an '86 300 is rated at 210 cfm. The stock '69 Autolite 1101 was rated at 210 also. The YF should have a better idle and mid range transition.

I hope that helps you.

Adios, David

 

[groomerguy]

Thanks for the response Dave. I installed the Pertronix ignition a few years ago. It was one of the first things I did when I started working on this unit. I run it on ported vacuum at 38 degrees total. I hope that's close. The engine seems to like it.

I'll leave the rockers alone.

When you talk about milling the head .025" anyway and then adding another .025" to bring the compression up, why do I need to mill it in the first place? Aside from taking a slice to clean up the surface, is there a head gasket thickness reason? Are the modern head gaskets thicker? What thickness should my head gasket be?

I've ordered a set of headers for it and will install the port divider. I was reading the thread on the divider and I don't see how it can hurt. I'll have the head off anyway so why not?

When I changed the carb I had to remove the adapter, tig the orginal carb holes shut, redrill them for the wider pattern and bore the center hole out. It seems strange to me that they'd both flow the same when one has a larger bottom hole. I know it made quite a difference as it seems to meter fuel more precisely. The throttle response is better and the fuel economy is up.

It'll be a couple of months before I do the work. I'll get some pics of the work. I read the sticky about posting pictures but I can't make it work so maybe I won't get them posted. No matter. Thanks again for the help.
Mel

 

[CZLN6]

Howdy Back Mel:

Right. The stock '69 head gasket used on all sixes was a steel shim type at .025" thick. They are no longer available. Aftermarket replacements are a composite type for better sealing, but are about .050" thick depending on brand. So the 1st .025" is to compensate for the thicker replacement head gasket.

The stock '69 250 had an advertized CR of 9:1. I don't believe it. Every 250 head and block I've been able to measure has a combustion chamber volume of 62 cc, and a deck clearance of .100" or more. In '72 the advertized CR dropped to 8:1, with no apparent change in piston, chamber volume or deck height. Clearly, 8:1 is more accurate. Milling an additional .025" will up your ratio to approximatel 8.5:1. Still within the range for using regular 85/87 octane gas. The extra CR will help with low end torque too.

I agree with you on the port divider with headers. I hand fitted mine to fit very tight. Then had it welded in as per the install instructions. I had my head flow tested before selecting the cam. While on the bench the tech used a short piece of exhaust pipe to hold to the outlet of the port as air was flowed from the chamber. Placing the pipe up to the port outlet increased flow. We tried the center siamized port both with and without the divider. The head pipe made little difference without the divider and port flow was down. With the divider in place flow was comparable with the other ports. While it is true that flow benches do not equate to HP, this was proof enough for me. I can't give a true before and after comparison because other changes were made at the same time, but I'm with you, I don't see how it can hurt, if properly installed. If it comes loose and rattles that would not be good. I also believe that anything that can help the exhaust side on these engines is good. Will you notice it in your application? Probably not, probably most noticeable with a performance cam and higher rpm applications, and then you'd need a dyno and individual cylinder temp reading to know.

Yea, I forgot about the carb bolt pattern differences. The CFM restriction is in the venturi, the throat above the butterfly. A larger throttle bore will not make up for a smaller venturi. Your combo of the YF and the Petronix are right on. You should notice much smoother engine operations.

PS- do take the time to remove the stock rocker shaft for a thorough cleaning on the inside. You will be amased at the buildup of crud and sludge. Also check both oil holes on each rocker to make sure they are fully open. We use a 1/8" drill to open the holes slightly.

 

[groomerguy]

That's all really good info David. If we were talking Chevys I could pass along lots of little tweaks and tricks but I'm really out of it with Fords. The rocker arm oiling tip is good along with a more through explaination about the head gasket differences and the head milling.
This engine rarely sees 3000 rpm. Even when we're running down the side of the road from one trail to another, or driving around a bridge we can't go across, we don't run it over 2800 rpm. It has cleated tracks and solid rubber tires so the ride is like an industrial vibrator. It also gets REALLY LOUD in the cab.
I do have a Holley 2 barrel in my shop. It looks to be pretty small but it might be worth putting on. Low end and maximum pulling power is the objective. Economy is also very important.
We groom at 2000 rpm and when it really pulls down we end up with it floored and struggling to pull through the heavy snow.
Do you think the timing is OK as I have it set up? I suspect the original timing was on vacuum and set at some initial point with the vacuum disconnected. Like I said, I put ours on ported vacuum and set it at 38 degrees when it was at 3000 rpm.
I'll get a better idea once the head is off and I can see the burn in the cylinders.
Have you ever used the ceramic coatings before? I recently modified my 1982 103" Harley Shovelhead and used the ceramic on the pistons tops, combustion chamber, exhaust valves and ports. I also put that slippery stuff on the piston skirts. Had it done through PolyDyn in Houston. It made quite a difference in the head temp. I have some jetting to do and then back to the dyno to see the difference. If you're interested I can give you the info.
Thanks again.

 

[CZLN6]

Howdy Mel:

Sorry to be late in replying. I've been off to NM for a few. Came home to 4" of new snow!!!!YUK!!!!!

I think the biggest difference you'll notice in the rig will be from the extra torque of the increase in CR.

If you have the time you might want to clean up the top of the pistons and the combustion chamber. That gives much the same effect as the coatings, but alot cheaper. Smoothed chambers will reflect heat and inhibit carbon deposits from forming. With an air cooled MC engine I think coatings are cheap insurance.

I forgot to ask what the Elevation is at your locale? If you're higher than 3,000- feet you might want to experiment with slightly more initial advance, which will add to your total. Be safe! Watch your temp guage and your spark plugs.

Adios, David

 

[groomerguy]

David,
We're less than 1,000 ft so I thought the 38 degrees total would work OK. I still haven't heard if using ported vacuum is common or advised. It's what I use on all my other "older" stuff that was originally manifold vacuum.
I do plan to take .050" off the head as you've advised. I was planning on cleaning up the chambers and exhaust ports.

My plan is to make sure the header would fit up first and while the valve job was being done I am going to weld a bung into the header so I can use my A/F meter. I have a nice one from Innovate Motorsports and it's a great tuning tool. Far more accurate than seat of the pants and trying to read plugs. On my 1968 P-30 bread truck I spent about 60 miles total changing jets, reading plugs and trying to get the mixture correct. After I installed the A/F unit I nailed it with just two short trips. Quite a difference.

Which tube is the O2 sensor placed in the late model fuel injected engines? I'll place it about 4" from the port, I just don't know which is the correct one. I could always put it in one of the collectors. On my Harley I put it in the front pipe about 4" from the port. It works great.

Thanks for the help!
Mel